Securities - Schedule of Fair Value of Available-for-Sale and Held-to-Maturity Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One Year or less	$ 87,447
Total available-for-sale securities, One to Five Years	118,637
Total available-for-sale securities, Five to Ten Years	95,372
Total available-for-sale securities, After Ten Years	58,055
Total available-for-sale securities, Mortgage-backed and Equity	1,294,753
Available-for-sale, Estimated Fair Value	1,654,264	917,424
Total held-to-maturity securities, One Year or less	2,852
Total held-to-maturity securities, One to Five Years	17,024
Total held-to-maturity securities, Five to Ten Years	266,172
Total held-to-maturity securities, After Ten Years	356,824
Total held-to-maturity securities, Mortgage-backed and Equity	129,971
Held-to-maturity securities, Fair value	772,843	619,617
Total securities, One Year or less	90,299
Total securities, One to Five Years	135,661
Total securities, Five to Ten Years	361,544
Total securities, After Ten Years, Fair value	414,879
Total securities, Mortgage- backed and Equity	1,424,724
Total securities, Fair value	2,427,107
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	1,283,445
Available-for-sale, Estimated Fair Value	1,283,445	701,113
Total held-to-maturity securities, Mortgage-backed and Equity	129,971
Held-to-maturity securities, Fair value	129,971	82,020
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	5,254
Total available-for-sale securities, One to Five Years	33,049
Total available-for-sale securities, Five to Ten Years	21,417
Total available-for-sale securities, After Ten Years	35,933
Available-for-sale, Estimated Fair Value	95,653	91,433
Total held-to-maturity securities, One Year or less	2,852
Total held-to-maturity securities, One to Five Years	17,024
Total held-to-maturity securities, Five to Ten Years	259,231
Total held-to-maturity securities, After Ten Years	356,824
Held-to-maturity securities, Fair value	635,931	530,801
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	82,193
Total available-for-sale securities, One to Five Years	62,312
Total available-for-sale securities, Five to Ten Years	33,658
Total available-for-sale securities, After Ten Years	4,927
Available-for-sale, Estimated Fair Value	183,090	25,996
Total held-to-maturity securities, Five to Ten Years	6,941
Held-to-maturity securities, Fair value	6,941	6,796
Obligations of Government Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, One to Five Years	23,276
Total available-for-sale securities, Five to Ten Years	40,297
Total available-for-sale securities, After Ten Years	17,195
Available-for-sale, Estimated Fair Value	80,768	87,736
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity [Line Items]
Total available-for-sale securities, Mortgage-backed and Equity	11,308
Available-for-sale, Estimated Fair Value	$ 11,308	$ 11,146