Investment Securities - Summary of Amortized Cost and Approximate Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 421,806	$ 414,928
Gross Unrealized Gains	3,778	4,106
Gross Unrealized Losses	(6,639)	(2,349)
Fair Value	418,945	416,685
Corporate notes
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	35,000	15,000
Gross Unrealized Gains	451	104
Gross Unrealized Losses	(129)	0
Fair Value	35,322	15,104
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	364,292	376,854
Gross Unrealized Gains	3,327	2,805
Gross Unrealized Losses	(1,859)	(2,348)
Fair Value	365,760	377,311
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	22,514	23,074
Gross Unrealized Gains	0	1,197
Gross Unrealized Losses	(4,651)	(1)
Fair Value	$ 17,863	$ 24,270