Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 19,804	$ 43,836
Held-to-maturity, Estimated Fair Value	474,215	495,082
Total, Amortized Cost	501,240	550,625
Total, Gross Unrealized Gains	5,524	7,381
Total, Gross Unrealized Losses	(12,745)	(19,088)
Total, Estimated Fair Value	494,019	538,918
Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	481,340	508,740
Held-to-maturity, Gross Unrealized Gains	5,524	5,430
Held-to-maturity, Gross Unrealized Losses	(12,649)	(19,088)
Held-to-maturity, Estimated Fair Value	474,215	495,082
Investment Securities [Member]
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost		41,885
Available-for-sale, Gross Unrealized Gains		1,951
Available-for-sale, Estimated Fair Value		43,836
Investment Securities [Member] | Equity Investments [Member]
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost		6,757
Available-for-sale, Gross Unrealized Gains		1,790
Available-for-sale, Estimated Fair Value		8,547
Investment Securities [Member] | U.S. Agency Obligations [Member]
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	19,900	35,128
Available-for-sale, Gross Unrealized Gains		161
Available-for-sale, Gross Unrealized Losses	(96)
Available-for-sale, Estimated Fair Value	19,804	35,289
Investment Securities [Member] | Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	155,223	159,190
Held-to-maturity, Gross Unrealized Gains	122	189
Held-to-maturity, Gross Unrealized Losses	(9,808)	(10,929)
Held-to-maturity, Estimated Fair Value	145,537	148,450
Investment Securities [Member] | Held-to-Maturity Securities [Member] | State and Municipal Obligations [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	13,829	21,784
Held-to-maturity, Gross Unrealized Gains	25	36
Held-to-maturity, Gross Unrealized Losses	(8)	(35)
Held-to-maturity, Estimated Fair Value	13,846	21,785
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	(9,750)	(10,750)
Held-to-maturity, Estimated Fair Value	45,250	44,250
Investment Securities [Member] | Held-to-Maturity Securities [Member] | U.S. Agency Obligations [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	86,394	82,406
Held-to-maturity, Gross Unrealized Gains	97	153
Held-to-maturity, Gross Unrealized Losses	(50)	(144)
Held-to-maturity, Estimated Fair Value	86,441	82,415
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	326,117	349,550
Held-to-maturity, Gross Unrealized Gains	5,402	5,241
Held-to-maturity, Gross Unrealized Losses	(2,841)	(8,159)
Held-to-maturity, Estimated Fair Value	328,678	346,632
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FHLMC [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	141,494	148,759
Held-to-maturity, Gross Unrealized Gains	609	447
Held-to-maturity, Gross Unrealized Losses	(1,659)	(4,552)
Held-to-maturity, Estimated Fair Value	140,444	144,654
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | FNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	184,003	200,070
Held-to-maturity, Gross Unrealized Gains	4,674	4,659
Held-to-maturity, Gross Unrealized Losses	(1,182)	(3,607)
Held-to-maturity, Estimated Fair Value	187,495	201,122
Mortgage-Backed Securities [Member] | Held-to-Maturity Securities [Member] | GNMA [Member]
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	620	721
Held-to-maturity, Gross Unrealized Gains	119	135
Held-to-maturity, Estimated Fair Value	$ 739	$ 856