Securities - Amortized Cost and Fair Value of Securities, by Contractual Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule Of Securities [Line Items]
Available for Sale, Due in one year or less, Amortized Cost
Available for Sale, Due from one to five years, Amortized Cost	283,913
Available for Sale, Due from five to ten years, Amortized Cost	47,102
Available for Sale, Due after ten years, Amortized Cost	6,885
Available for Sale, with contractual maturities, Amortized Cost	337,900
Total securities available for sale, Amortized Cost	1,449,263	1,169,482
Available for Sale, Due in one year or less, Fair Value
Available for Sale, Due from one to five years, Fair Value	282,767
Available for Sale, Due from five to ten years, Fair Value	46,344
Available for Sale, Due after ten years, Fair Value	6,303
Available for Sale, with contractual maturities, Fair Value	335,414
Total available for sale Securities, Fair Value	1,439,735	1,141,650
Held to Maturity, Due in one year or less, Amortized Cost	3,711
Held to Maturity, Due from one to five years, Amortized Cost	90,393
Held to Maturity, Due from five to ten years, Amortized Cost	79,109
Held to Maturity, Due after ten years, Amortized Cost	78,836
Securities Held To Maturity, with contractual maturities, Amortized Cost	252,049
Held to Maturity, with contractual maturities, Amortized Cost	1,475,552	1,199,169
Held to Maturity, Due in one year or less, Fair Value	3,741
Held to Maturity, Due from one to five years, Fair Value	89,667
Held to Maturity, Due from five to ten years, Fair Value	80,905
Held to Maturity, Due after ten years, Fair Value	80,462
Securities Held To Maturity, with contractual maturities, Fair Value	254,775
Held to Maturity, with contractual maturities, Fair Value	1,480,229	1,189,563
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	492,456	247,880
Total available for sale Securities, Fair Value	496,483
Held to Maturity, with contractual maturities, Amortized Cost	709,123	628,681
Held to Maturity, with contractual maturities, Fair Value	720,759	634,930
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	608,284	511,098
Total available for sale Securities, Fair Value	596,934
Held to Maturity, with contractual maturities, Amortized Cost	494,791	385,408
Held to Maturity, with contractual maturities, Fair Value	485,068	370,328
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	1,530	1,747
Total available for sale Securities, Fair Value	1,547
Held to Maturity, with contractual maturities, Amortized Cost	4,630	6,852
Held to Maturity, with contractual maturities, Fair Value	4,645	6,892
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	8,062
Total available for sale Securities, Fair Value	8,037
Held to Maturity, with contractual maturities, Amortized Cost	14,959	2,241
Held to Maturity, with contractual maturities, Fair Value	14,982	2,328
Equity Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	1,031	1,444
Total available for sale Securities, Fair Value	1,320
Held to Maturity, with contractual maturities, Amortized Cost
Held to Maturity, with contractual maturities, Fair Value