Securities - Schedule of Fair Value of Available-for-Sale and Held-to-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	$ 10,062
Total available-for-sale securities, One to Five Years	64,054
Total available-for-sale securities, Five to Ten Years	56,337
Total available-for-sale securities, After Ten Years	61,661
Total available-for-sale securities, Mortgage-backed and Equity	1,049,062
Available-for-sale, Estimated Fair Value	1,241,176	$ 1,403,069
Total held-to-maturity securities, One Year or less	735
Total held-to-maturity securities, One to Five Years	70,956
Total held-to-maturity securities, Five to Ten Years	441,587
Total held-to-maturity securities, After Ten Years	336,675
Total held-to-maturity securities, Mortgage-backed and Equity	226,837
Held-to-maturity securities, Fair value	1,076,790	1,038,207
Total, One Year or less	10,797
Total, One to Five Years	135,010
Total, Five to Ten Years	497,924
Total, After Ten Years, Fair value	398,336
Total, Mortgage- backed and Equity	1,275,899
Total, Fair value	2,317,966	2,441,276
U.S. Government Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	2,000
Total available-for-sale securities, One to Five Years	11,896
Total available-for-sale securities, Five to Ten Years	16,729
Total available-for-sale securities, After Ten Years	14,525
Total available-for-sale securities, Mortgage-backed and Equity	8,893
Available-for-sale, Estimated Fair Value	54,043	83,505
Total held-to-maturity securities, Mortgage-backed and Equity	12,980
Held-to-maturity securities, Fair value	12,980
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, Mortgage-backed and Equity	1,035,099
Available-for-sale, Estimated Fair Value	1,035,099	1,176,080
Total held-to-maturity securities, Mortgage-backed and Equity	213,857
Held-to-maturity securities, Fair value	213,857	216,327
Obligations of State and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	8,062
Total available-for-sale securities, One to Five Years	21,835
Total available-for-sale securities, Five to Ten Years	36,571
Total available-for-sale securities, After Ten Years	45,195
Available-for-sale, Estimated Fair Value	111,663	80,265
Total held-to-maturity securities, One Year or less	735
Total held-to-maturity securities, One to Five Years	69,980
Total held-to-maturity securities, Five to Ten Years	407,752
Total held-to-maturity securities, After Ten Years	336,675
Held-to-maturity securities, Fair value	815,142	787,434
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One to Five Years	30,323
Total available-for-sale securities, Five to Ten Years	3,037
Total available-for-sale securities, After Ten Years	1,941
Available-for-sale, Estimated Fair Value	35,301	58,593
Total held-to-maturity securities, One to Five Years	976
Total held-to-maturity securities, Five to Ten Years	33,835
Held-to-maturity securities, Fair value	34,811	34,446
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, Mortgage-backed and Equity	5,070
Available-for-sale, Estimated Fair Value	$ 5,070	$ 4,626