Interest-bearing Time Deposits in Banks and Securities - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015 Investment	Mar. 31, 2014	Mar. 31, 2015	Dec. 31, 2014
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing deposits in banks	$ 9,170,000	$ 30,026,000	$ 9,170,000	$ 17,002,000
Time deposits with balances greater than $250,000, FDIC insured limit	983,000		983,000	3,195,000
Held-to-maturity securities	310,000	582,000	310,000	441,000
Number of investment positions	76		76
Securities pledged as collateral	1,602,606,000		1,602,606,000
Sales of investment securities available-for-sale	1,129,000	904,000
Gross realized gains from security sales	5,000	1,000
Gross realized losses from security sales	0	5,000
Minimum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years	1 year
Time deposits balances, FDIC insured limit			$ 250,000	$ 250,000
Maximum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years	4 years