Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 30,108	$ 19,804
Held-to-maturity, Amortized Cost	403,838	481,340
Held-to-maturity, Estimated Fair Value	400,852	474,215
Total, Amortized Cost	433,735	501,240
Total, Gross Unrealized Gains	5,292	5,524
Total, Gross Unrealized Losses	(8,067)	(12,745)
Total, Estimated Fair Value	430,960	494,019
Corporate Debt Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	55,000
Held-to-maturity, Estimated Fair Value	48,072
Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	403,838	481,340
Held-to-maturity, Gross Unrealized Gains	5,081	5,524
Held-to-maturity, Gross Unrealized Losses	(8,067)	(12,649)
Held-to-maturity, Estimated Fair Value	400,852	474,215
Investment Securities [Member] | U.S. Agency Obligations [Member]
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	29,897	19,900
Available-for-sale, Gross Unrealized Gains	211
Available-for-sale, Gross Unrealized Losses		(96)
Available-for-sale, Estimated Fair Value	30,108	19,804
Investment Securities [Member] | Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	118,711	155,223
Held-to-maturity, Gross Unrealized Gains	197	122
Held-to-maturity, Gross Unrealized Losses	(6,933)	(9,808)
Held-to-maturity, Estimated Fair Value	111,975	145,537
Investment Securities [Member] | Held-to-Maturity Securities [Member] | U.S. Agency Obligations [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	55,338	86,394
Held-to-maturity, Gross Unrealized Gains	172	97
Held-to-maturity, Gross Unrealized Losses	(2)	(50)
Held-to-maturity, Estimated Fair Value	55,508	86,441
Investment Securities [Member] | Held-to-Maturity Securities [Member] | State and Municipal Obligations [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	8,373	13,829
Held-to-maturity, Gross Unrealized Gains	25	25
Held-to-maturity, Gross Unrealized Losses	(3)	(8)
Held-to-maturity, Estimated Fair Value	8,395	13,846
Investment Securities [Member] | Held-to-Maturity Securities [Member] | Corporate Debt Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	55,000	55,000
Held-to-maturity, Gross Unrealized Losses	(6,928)	(9,750)
Held-to-maturity, Estimated Fair Value	48,072	45,250
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	285,127	326,117
Held-to-maturity, Gross Unrealized Gains	4,884	5,402
Held-to-maturity, Gross Unrealized Losses	(1,134)	(2,841)
Held-to-maturity, Estimated Fair Value	288,877	328,678
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FHLMC [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	126,187	141,494
Held-to-maturity, Gross Unrealized Gains	867	609
Held-to-maturity, Gross Unrealized Losses	(658)	(1,659)
Held-to-maturity, Estimated Fair Value	126,396	140,444
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	158,417	184,003
Held-to-maturity, Gross Unrealized Gains	3,913	4,674
Held-to-maturity, Gross Unrealized Losses	(476)	(1,182)
Held-to-maturity, Estimated Fair Value	161,854	187,495
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | GNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	523	620
Held-to-maturity, Gross Unrealized Gains	104	119
Held-to-maturity, Estimated Fair Value	$ 627	$ 739