Allowance for Loan Losses - Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,159,198	$ 976,224
Percentage of Total	100.00%	100.00%
Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,154,983	$ 971,798
Percentage of Total	99.60%	99.50%
Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 4,215	$ 4,426
Percentage of Total	0.40%	0.50%
Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 599,502	$ 489,586
Percentage of Total	100.00%	100.00%
Residential Mortgage [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 597,492	$ 487,250
Percentage of Total	99.70%	99.50%
Residential Mortgage [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 2,010	$ 2,336
Percentage of Total	0.30%	0.50%
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 457,813	$ 418,563
Percentage of Total	100.00%	100.00%
Home Equity [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 455,787	$ 416,504
Percentage of Total	99.60%	99.50%
Home Equity [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 2,026	$ 2,059
Percentage of Total	0.40%	0.50%
Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 94,437	$ 65,567
Percentage of Total	100.00%	100.00%
Consumer Borrower [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 94,258	$ 65,536
Percentage of Total	99.80%	99.90%
Consumer Borrower [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 179	$ 31
Percentage of Total	0.20%	0.10%
Consumer Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 7,446	$ 2,508
Percentage of Total	100.00%	100.00%
Consumer Construction [Member] | Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 7,446	$ 2,508
Percentage of Total	100.00%	100.00%