Quarterly report pursuant to Section 13 or 15(d)

BORROWINGS (Narrative) (Details)

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BORROWINGS (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Jan. 01, 2018
Jan. 01, 2014
Aug. 23, 2012
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Subordinated Borrowing [Line Items]              
Remaining available balance for the federal funds lines       $ 227,000   $ 227,000  
Remaining available balance of alternate line of credit       25,000   25,000  
Maximum collateral dependent line of credit with the FHLB       3,900,000   2,700,000  
Subordinated debt       158,500   150,000 $ 150,000
Prepayment penalty     $ 19,600        
Prepayment penalty amortization expense       481 $ 470    
Trust Preferred Capital Notes              
Subordinated Borrowing [Line Items]              
Trust preferred capital notes principal balance       145,500      
Remaining fair value discount on acquired notes       16,200      
Subordinated Debt              
Subordinated Borrowing [Line Items]              
Remaining fair value discount on acquired notes       $ 1,700   $ 1,800  
Fixed interest rate on debt instrument             5.00%
Subordinated Debt | LIBOR              
Subordinated Borrowing [Line Items]              
Spread on LIBOR       3.175%      
Acquisitions, Prior To 2006              
Subordinated Borrowing [Line Items]              
Trust preferred capital notes principal balance       $ 58,500      
StellarOne              
Subordinated Borrowing [Line Items]              
Long-term FHLB borrowings   $ 70,000   10,000      
Remaining fair value premium on acquired FHLB advances       74      
StellarOne | Trust Preferred Capital Notes              
Subordinated Borrowing [Line Items]              
Trust preferred capital notes principal balance   $ 32,000          
Xenith              
Subordinated Borrowing [Line Items]              
Trust preferred capital notes principal balance $ 55,000            
Subordinated debt 8,500            
Xenith | Trust Preferred Capital Notes              
Subordinated Borrowing [Line Items]              
Remaining fair value discount on acquired notes 9,900            
Xenith | Subordinated Debt              
Subordinated Borrowing [Line Items]              
Remaining fair value discount on acquired notes $ 259     $ 233      
Fixed interest rate on debt instrument 6.75%