Securities - Amortized Cost and Fair Value of Securities, by Contractual Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule Of Securities [Line Items]
Available for Sale, Due in one year or less, Amortized Cost	$ 4,990
Available for Sale, Due from one to five years, Amortized Cost	438,979
Available for Sale, Due from five to ten years, Amortized Cost	10,014
Available for Sale, Due after ten years, Amortized Cost	6,892
Available for Sale, with contractual maturities, Amortized Cost	460,875
Total securities available for sale, Amortized Cost	1,617,439	$ 1,534,771
Available for Sale, Due in one year or less, Fair Value	5,040
Available for Sale, Due from one to five years, Fair Value	440,208
Available for Sale, Due from five to ten years, Fair Value	10,304
Available for Sale, Due after ten years, Fair Value	6,150
Available for Sale, with contractual maturities, Fair Value	461,702
Total available for sale Securities, Fair Value	1,618,620	1,534,065
Held to Maturity, Due in one year or less, Amortized Cost	1,010
Held to Maturity, Due from one to five years, Amortized Cost	140,854
Held to Maturity, Due from five to ten years, Amortized Cost	72,031
Held to Maturity, Due after ten years, Amortized Cost	127,731
Securities Held To Maturity, with contractual maturities, Amortized Cost	341,626
Held to Maturity, with contractual maturities, Amortized Cost	1,518,060	1,453,355
Held to Maturity, Due in one year or less, Fair Value	1,017
Held to Maturity, Due from one to five years, Fair Value	141,072
Held to Maturity, Due from five to ten years, Fair Value	73,550
Held to Maturity, Due after ten years, Fair Value	126,192
Securities Held To Maturity, with contractual maturities, Fair Value	341,831
Held to Maturity, with contractual maturities, Fair Value	1,526,040	1,468,258
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	588,103	546,572
Total available for sale Securities, Fair Value	593,756
Held to Maturity, with contractual maturities, Amortized Cost	670,817	677,169
Held to Maturity, with contractual maturities, Fair Value	682,792	693,535
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	561,212	580,601
Total available for sale Securities, Fair Value	555,597
Held to Maturity, with contractual maturities, Amortized Cost	484,832	501,965
Held to Maturity, with contractual maturities, Fair Value	480,390	496,494
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	1,270	1,414
Total available for sale Securities, Fair Value	1,279
Held to Maturity, with contractual maturities, Amortized Cost	3,375	4,285
Held to Maturity, with contractual maturities, Fair Value	3,386	4,313
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	5,004	7,891
Total available for sale Securities, Fair Value	5,006
Held to Maturity, with contractual maturities, Amortized Cost	17,410	17,560
Held to Maturity, with contractual maturities, Fair Value	17,641	17,739
Equity Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	975	$ 1,031
Total available for sale Securities, Fair Value	$ 1,280