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Floating Rate Capital Debt - Narrative (Detail) - USD ($)
1 Months Ended 12 Months Ended
Apr. 30, 1997
Jan. 31, 1997
Dec. 31, 2018
Debt Instrument [Line Items]      
Percentage of securities eligible for Tier 2 capital treatment     40.00%
Securities eligible for Tier 2 capital treatment, decline rate     10.00%
NTC Capital I Subordinated Debentures due January 15, 2027      
Debt Instrument [Line Items]      
Floating rate capital securities, issued   $ 150,000,000  
Floating rate capital securities, due date Jan. 15, 2027    
Liquidation amount per Security   $ 1,000  
NTC Capital I Subordinated Debentures due January 15, 2027 | Group Two      
Debt Instrument [Line Items]      
Debt instrument, basis spread on variable rate   0.52%  
NTC Capital II Subordinated Debentures due April 15, 2027      
Debt Instrument [Line Items]      
Floating rate capital securities, issued $ 120,000,000    
Liquidation amount per Security $ 1,000    
NTC Capital II Subordinated Debentures due April 15, 2027 | Group Two      
Debt Instrument [Line Items]      
Debt instrument, basis spread on variable rate 0.59%    
London Interbank Offered Rate (LIBOR) | NTC Capital I Subordinated Debentures due January 15, 2027      
Debt Instrument [Line Items]      
Debt instrument, basis spread on variable rate     0.605%
London Interbank Offered Rate (LIBOR) | NTC Capital II Subordinated Debentures due April 15, 2027      
Debt Instrument [Line Items]      
Debt instrument, basis spread on variable rate     0.679%