Allowance for Loan Losses - Recorded Investment in Consumer Loan Classes by Performing and Nonperforming Status (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,192,135	$ 976,224
Percentage of Total	100.00%	100.00%
Performing Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 1,186,624	$ 971,798
Percentage of Total	99.50%	99.50%
Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 5,511	$ 4,426
Percentage of Total	0.50%	0.50%
Residential Mortgage [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 625,251	$ 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	$ 622,009	$ 487,250
Percentage of Total	99.50%	99.50%
Residential Mortgage [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 3,242	$ 2,336
Percentage of Total	0.50%	0.50%
Home Equity [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 467,698	$ 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	$ 465,551	$ 416,504
Percentage of Total	99.50%	99.50%
Home Equity [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 2,147	$ 2,059
Percentage of Total	0.50%	0.50%
Consumer Borrower [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 91,122	$ 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	$ 91,000	$ 65,536
Percentage of Total	99.90%	99.90%
Consumer Borrower [Member] | Nonperforming Financing Receivable [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 122	$ 31
Percentage of Total	0.10%	0.10%
Consumer Construction [Member]
Financing Receivable, Recorded Investment [Line Items]
Recorded investment in consumer loan	$ 8,064	$ 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	$ 8,064	$ 2,508
Percentage of Total	100.00%	100.00%