Securities - Schedule of Fair Value of Available-for-Sale and Held-to-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	$ 39,014
Total available-for-sale securities, One to Five Years	91,686
Total available-for-sale securities, Five to Ten Years	102,340
Total available-for-sale securities, After Ten Years	54,340
Total available-for-sale securities, Mortgage-backed and Equity	1,841,904
Available-for-sale, Estimated Fair Value	2,129,284	$ 2,114,129
Total held-to-maturity securities, One Year or less	15,652
Total held-to-maturity securities, One to Five Years	126,637
Total held-to-maturity securities, Five to Ten Years	358,156
Total held-to-maturity securities, After Ten Years	274,016
Total held-to-maturity securities, Mortgage-backed and Equity	147,073
Held-to-maturity securities, Fair value	921,534	1,020,743
Total, One Year or less	54,666
Total, One to Five Years	218,323
Total, Five to Ten Years	460,496
Total, After Ten Years, Fair value	328,356
Total, Mortgage- backed and Equity	1,988,977
Total, Fair value	3,050,818	3,134,872
US Treasury Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	29,781
Available-for-sale, Estimated Fair Value	29,781	19,878
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	498
Total available-for-sale securities, One to Five Years	6,048
Total available-for-sale securities, Five to Ten Years	18,472
Total available-for-sale securities, After Ten Years	9,363
Total available-for-sale securities, Mortgage-backed and Equity	100,206
Available-for-sale, Estimated Fair Value	134,587	141,652
Total held-to-maturity securities, Mortgage-backed and Equity	9,952
Held-to-maturity securities, Fair value	9,952	10,500
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, Mortgage-backed and Equity	1,563,042
Available-for-sale, Estimated Fair Value	1,563,042	1,561,255
Total held-to-maturity securities, Mortgage-backed and Equity	137,121
Held-to-maturity securities, Fair value	137,121	144,334
Commercial Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Sponsored Entities and Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, Mortgage-backed and Equity	178,656
Available-for-sale, Estimated Fair Value	178,656	168,972
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	8,108
Total available-for-sale securities, One to Five Years	53,422
Total available-for-sale securities, Five to Ten Years	76,671
Total available-for-sale securities, After Ten Years	44,977
Available-for-sale, Estimated Fair Value	183,178	185,114
Total held-to-maturity securities, One Year or less	15,652
Total held-to-maturity securities, One to Five Years	115,724
Total held-to-maturity securities, Five to Ten Years	334,357
Total held-to-maturity securities, After Ten Years	274,016
Held-to-maturity securities, Fair value	739,749	833,279
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	627
Total available-for-sale securities, One to Five Years	32,216
Total available-for-sale securities, Five to Ten Years	7,197
Available-for-sale, Estimated Fair Value	40,040	37,258
Total held-to-maturity securities, One to Five Years	10,913
Total held-to-maturity securities, Five to Ten Years	23,799
Held-to-maturity securities, Fair value	$ 34,712	$ 32,630