Allowance for Loan Losses - Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,190,735	$ 976,224
% of Total	100.00%	100.00%
Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,182,104	$ 971,798
% of Total	99.30%	99.50%
Non Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 8,631	$ 4,426
% of Total	0.70%	0.50%
Residential Mortgage
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 639,372	$ 489,586
% of Total	100.00%	100.00%
Residential Mortgage | Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 633,743	$ 487,250
% of Total	99.10%	99.50%
Residential Mortgage | Non Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 5,629	$ 2,336
% of Total	0.90%	0.50%
Home Equity
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 470,845	$ 418,563
% of Total	100.00%	100.00%
Home Equity | Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 467,943	$ 416,504
% of Total	99.40%	99.50%
Home Equity | Non Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 2,902	$ 2,059
% of Total	0.60%	0.50%
Installment and Other Consumer
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 73,939	$ 65,567
% of Total	100.00%	100.00%
Installment and Other Consumer | Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 73,839	$ 65,536
% of Total	99.80%	99.90%
Installment and Other Consumer | Non Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 100	$ 31
% of Total	0.20%	0.10%
Consumer Construction
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 6,579	$ 2,508
% of Total	100.00%	100.00%
Consumer Construction | Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 6,579	$ 2,508
% of Total	100.00%	100.00%
Consumer Construction | Non Performing Consumer Loan
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 0	$ 0
% of Total	0.00%	0.00%