Securities - Amortized Cost and Fair Value of Securities, by Contractual Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule Of Securities [Line Items]
Available for Sale, Due in one year or less, Amortized Cost	$ 14,985
Available for Sale, Due from one to five years, Amortized Cost	460,162
Available for Sale, Due from five to ten years, Amortized Cost	27,882
Available for Sale, Due after ten years, Amortized Cost	7,283
Available for Sale, with contractual maturities, Amortized Cost	510,312
Total securities available for sale, Amortized Cost	2,107,479	$ 1,636,526
Available for Sale, Due in one year or less, Fair Value	15,061
Available for Sale, Due from one to five years, Fair Value	463,729
Available for Sale, Due from five to ten years, Fair Value	28,135
Available for Sale, Due after ten years, Fair Value	6,522
Available for Sale, with contractual maturities, Fair Value	513,447
Total securities available for sale, Fair Value	2,133,662	1,630,567
Held to Maturity, Due in one year or less, Amortized Cost	11,478
Held to Maturity, Due from one to five years, Amortized Cost	234,393
Held to Maturity, Due from five to ten years, Amortized Cost	48,491
Held to Maturity, Due after ten years, Amortized Cost	220,559
Securities Held To Maturity, with contractual maturities, Amortized Cost	514,921
Held to Maturity, with contractual maturities, Amortized Cost	2,064,305	1,637,061
Held to Maturity, Due in one year or less, Fair Value	11,502
Held to Maturity, Due from one to five years, Fair Value	236,292
Held to Maturity, Due from five to ten years, Fair Value	49,991
Held to Maturity, Due after ten years, Fair Value	228,444
Securities Held To Maturity, with contractual maturities, Fair Value	526,229
Held to Maturity, with contractual maturities, Fair Value	2,104,782	1,643,416
Agency Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	1,095,826	703,069
Total securities available for sale, Fair Value	1,114,877
Held to Maturity, with contractual maturities, Amortized Cost	882,467	709,970
Held to Maturity, with contractual maturities, Fair Value	904,889	718,652
Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	495,943	503,328
Total securities available for sale, Fair Value	499,647
Held to Maturity, with contractual maturities, Amortized Cost	614,480	499,694
Held to Maturity, with contractual maturities, Fair Value	619,531	492,840
Non-Agency Collateralized Mortgage Obligations [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	1,034	1,177
Total securities available for sale, Fair Value	978
Held to Maturity, with contractual maturities, Amortized Cost	2,143	2,681
Held to Maturity, with contractual maturities, Fair Value	2,144	2,695
Commercial Mortgage-Backed Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	3,389	4,299
Total securities available for sale, Fair Value	3,393
Held to Maturity, with contractual maturities, Amortized Cost	50,294	51,258
Held to Maturity, with contractual maturities, Fair Value	51,989	51,114
Equity Securities [Member]
Schedule Of Securities [Line Items]
Total securities available for sale, Amortized Cost	975	$ 975
Total securities available for sale, Fair Value	$ 1,320