Fair Value Measurement - Additional Information (Detail) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers between any levels	$ 0		$ 0
Carrying value of loans fully charged-off		$ 0
Securities held to maturity	751,051,000	751,051,000	676,426,000
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities held to maturity	$ 0	$ 0	$ 0