Allowance for Loan Losses - Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,034,066	$ 976,224
% of Total	100.00%	100.00%
Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	1,029,841	971,798
% of Total	99.60%	99.50%
Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	4,225	4,426
% of Total	0.40%	0.50%
Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	521,506	489,586
% of Total	100.00%	100.00%
Residential Mortgage [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	519,280	487,250
% of Total	99.60%	99.50%
Residential Mortgage [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	2,226	2,336
% of Total	0.40%	0.50%
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	442,396	418,563
% of Total	100.00%	100.00%
Home Equity [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	440,410	416,504
% of Total	99.60%	99.50%
Home Equity [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	1,986	2,059
% of Total	0.40%	0.50%
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	65,754	65,567
% of Total	100.00%	100.00%
Installment and Other Consumer [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	65,741	65,536
% of Total	100.00%	99.90%
Installment and Other Consumer [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	13	31
% of Total	0.00%	0.10%
Consumer Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	4,410	2,508
% of Total	100.00%	100.00%
Consumer Construction [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	4,410	2,508
% of Total	100.00%	100.00%
Consumer Construction [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 0	$ 0
% of Total	0.00%	0.00%