Securities - Amortized Cost and Fair Value of Securities, by Contractual Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Securities [Line Items]
Available for sale, due in one year or less, amortized cost	$ 81,819
Available for sale, due from one to five years, amortized cost	278,153
Available for sale, due from five to ten years, amortized cost	10,892
Available for sale, due after ten years, amortized cost	2,967
Available for sale, with contractual maturities, amortized cost	373,831
Total securities available for sale, amortized cost	2,802,621	$ 2,260,021
Available for sale, due in one year or less, fair value	81,594
Available for sale, due from one to five years, fair value	274,516
Available for sale, due from five to ten years, fair value	10,795
Available for sale, due after ten years, fair value	2,800
Available for sale, with contractual maturities, fair value	369,705
Total securities available for sale, fair value	2,764,562	2,231,987
Held to maturity, due in one year or less, amortized cost	30,438
Held to maturity, due from one to five years, amortized cost	225,054
Held to maturity, due from five to ten years, amortized cost	81,650
Held to maturity, due after ten years, amortized cost	827,392
Securities held to maturity, with contractual maturities, amortized cost	1,164,534
Held to maturity, with contractual maturities, amortized cost	3,242,268	2,337,342
Held to maturity, due in one year or less, fair value	30,254
Held to maturity, due from one to five years, fair value	220,921
Held to maturity, due from five to ten years, fair value	82,672
Held to maturity, due after ten years, fair value	832,605
Securities held to maturity, with contractual maturities, fair value	1,166,452
Held to maturity, with contractual maturities, fair value	3,218,379	2,294,777
Agency mortgage-backed securities
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	1,615,168	1,267,535
Total securities available for sale, fair value	1,598,874	1,252,798
Held to maturity, with contractual maturities, amortized cost	1,219,802
Held to maturity, with contractual maturities, fair value	1,214,219
Agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	813,034	546,659
Total securities available for sale, fair value	794,957	535,974
Held to maturity, with contractual maturities, amortized cost	777,146
Held to maturity, with contractual maturities, fair value	757,083
Non-agency collateralized mortgage obligations
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	1	891
Total securities available for sale, fair value	1	897
Held to maturity, with contractual maturities, amortized cost	0
Held to maturity, with contractual maturities, fair value	0
Commercial mortgage-backed securities
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	0	1,292
Total securities available for sale, fair value	0	1,291
Held to maturity, with contractual maturities, amortized cost	80,786
Held to maturity, with contractual maturities, fair value	80,625
Equity securities
Schedule Of Securities [Line Items]
Total securities available for sale, amortized cost	587	273
Total securities available for sale, fair value	1,025	$ 640
Held to maturity, with contractual maturities, amortized cost	0
Held to maturity, with contractual maturities, fair value	$ 0