Interest-bearing Time Deposits in Banks and Securities - Additional Information (Detail)				3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Investment	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) Investment	Sep. 30, 2014 USD ($)
Interest Bearing Time Deposits In Banks And Securities [Line Items]
INTEREST-BEARING TIME DEPOSITS IN BANKS	$ 4,491,000	$ 17,002,000	$ 19,234,000	$ 4,491,000	$ 19,234,000	$ 4,491,000	$ 19,234,000
Time deposits with balances greater than $250,000, FDIC insured limit	489,000	3,195,000	3,693,000	489,000	3,693,000	489,000	3,693,000
Held-to-maturity securities	$ 286,000	441,000	554,000	$ 286,000	554,000	$ 286,000	554,000
Number of investment positions | Investment	80			80		80
Securities pledged as collateral	$ 1,667,548,000			$ 1,667,548,000		$ 1,667,548,000
Sales of investment securities available-for-sale				27,910,000	1,839,000	34,541,000	3,590,000
Gross realized gains from security sales				142,000	1,000	390,000	1,000
Gross realized losses from security sales or calls				$ 6,000	$ 0	$ 10,000	$ 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	$ 250,000
Maximum [Member]
Interest Bearing Time Deposits In Banks And Securities [Line Items]
Interest-bearing time deposits, maturity period, years						4 years