Securities - Schedule of Fair Value of Available-for-Sale and Held-to-Maturity Debt Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	$ 39,802
Total available-for-sale securities, One to Five Years	63,497
Total available-for-sale securities, Five to Ten Years	66,729
Total available-for-sale securities, After Ten Years	49,099
Total available-for-sale securities, Mortgage-backed	1,577,444
Available-for-sale, Estimated Fair Value	1,796,571	$ 1,261,865
Total held-to-maturity securities, One Year or less	6,410
Total held-to-maturity securities, One to Five Years	130,695
Total held-to-maturity securities, Five to Ten Years	418,288
Total held-to-maturity securities, After Ten Years	293,678
Total held-to-maturity securities, Mortgage-backed	167,040
Held-to-maturity securities, Fair value	1,016,111	1,023,784
Total, One Year or less	46,212
Total, One to Five Years	194,192
Total, Five to Ten Years	485,017
Total, After Ten Years, Fair value	342,777
Total, Mortgage-backed	1,744,484
Total, Fair value	2,812,682	2,285,649
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	9,913
Available-for-sale, Estimated Fair Value	9,913
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	10,459
Total available-for-sale securities, One to Five Years	6,302
Total available-for-sale securities, Five to Ten Years	13,739
Total available-for-sale securities, After Ten Years	6,802
Total available-for-sale securities, Mortgage-backed	63,380
Available-for-sale, Estimated Fair Value	100,682	71,843
Total held-to-maturity securities, Mortgage-backed	11,487
Held-to-maturity securities, Fair value	11,487	11,140
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	1,373,417
Available-for-sale, Estimated Fair Value	1,373,417	934,922
Total held-to-maturity securities, Mortgage-backed	155,553
Held-to-maturity securities, Fair value	155,553	167,960
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	140,647
Available-for-sale, Estimated Fair Value	140,647	114,867
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	9,629
Total available-for-sale securities, One to Five Years	23,831
Total available-for-sale securities, Five to Ten Years	51,042
Total available-for-sale securities, After Ten Years	42,297
Available-for-sale, Estimated Fair Value	126,799	104,830
Total held-to-maturity securities, One Year or less	6,410
Total held-to-maturity securities, One to Five Years	123,247
Total held-to-maturity securities, Five to Ten Years	393,073
Total held-to-maturity securities, After Ten Years	293,678
Held-to-maturity securities, Fair value	816,408	810,410
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	9,801
Total available-for-sale securities, One to Five Years	33,364
Total available-for-sale securities, Five to Ten Years	1,948
Available-for-sale, Estimated Fair Value	45,113	35,403
Total held-to-maturity securities, One to Five Years	7,448
Total held-to-maturity securities, Five to Ten Years	25,215
Held-to-maturity securities, Fair value	$ 32,663	$ 34,274