Allowance for Loan Losses - Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 976,224	$ 972,319
% of Total	100.00%	100.00%
Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	971,798	966,653
% of Total	99.50%	99.40%
Non Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	4,426	5,666
% of Total	0.50%	0.60%
Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	489,586	487,092
% of Total	100.00%	100.00%
Residential Mortgage [Member] | Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	487,250	483,754
% of Total	99.50%	99.30%
Residential Mortgage [Member] | Non Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	2,336	3,338
% of Total	0.50%	0.70%
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	418,563	414,195
% of Total	100.00%	100.00%
Home Equity [Member] | Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	416,504	411,904
% of Total	99.50%	99.40%
Home Equity [Member] | Non Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	2,059	2,291
% of Total	0.50%	0.60%
Installment and Other Consumer [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	65,567	67,883
% of Total	100.00%	100.00%
Installment and Other Consumer [Member] | Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	65,536	67,846
% of Total	99.90%	99.90%
Installment and Other Consumer [Member] | Non Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	31	37
% of Total	0.10%	0.10%
Consumer Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	2,508	3,149
% of Total	100.00%	100.00%
Consumer Construction [Member] | Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	2,508	3,149
% of Total	100.00%	100.00%
Consumer Construction [Member] | Non Performing Consumer Loan [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 0	$ 0
% of Total	0.00%	0.00%