Securities - Schedule of Fair Value of Available-for-Sale and Held-to-Maturity Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Available For Sale Or Held To Maturity Securities [Line Items]
Total available-for-sale securities, One Year or less	$ 14,538
Total available-for-sale securities, One to Five Years	77,871
Total available-for-sale securities, Five to Ten Years	81,308
Total available-for-sale securities, After Ten Years	71,938
Total available-for-sale securities, Mortgage-backed and Equity	725,523
Available-for-sale, Estimated Fair Value	971,178	1,021,244
Total held-to-maturity securities, One Year or less, Fair value	2,715
Total held-to-maturity securities, One to Five Years	10,594
Total held-to-maturity securities, Five to Ten years	117,970
Total held-to-maturity securities, After Ten Years	364,044
Total held-to-maturity securities, Mortgage-backed and Equity	119,880
Held-to-maturity securities, Fair value	615,203	639,273
Total securities, One Year or less	17,253
Total securities, One to Five Years	88,465
Total securities, Five to Ten Years, Fair value	199,278
Total securities, After Ten Years, Fair value	435,982
Total securities, Mortgage- backed and Equity	845,403
Total securities, Fair value	1,586,381
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Schedule Of Available For Sale Or Held To Maturity Securities [Line Items]
Total available-for-sale securities, One Year or less
Total available-for-sale securities, One to Five Years
Total available-for-sale securities, Five to Ten Years
Total available-for-sale securities, After Ten Years
Total available-for-sale securities, Mortgage-backed and Equity	714,416
Available-for-sale, Estimated Fair Value	714,416	732,210
Total held-to-maturity securities, One Year or less, Fair value
Total held-to-maturity securities, One to Five Years
Total held-to-maturity securities, Five to Ten years
Total held-to-maturity securities, After Ten Years
Total held-to-maturity securities, Mortgage-backed and Equity	119,880
Held-to-maturity securities, Fair value	119,880	159,213
Obligations of State and Political Subdivisions [Member]
Schedule Of Available For Sale Or Held To Maturity Securities [Line Items]
Total available-for-sale securities, One Year or less	8,397
Total available-for-sale securities, One to Five Years	43,979
Total available-for-sale securities, Five to Ten Years	29,347
Total available-for-sale securities, After Ten Years	50,491
Total available-for-sale securities, Mortgage-backed and Equity
Available-for-sale, Estimated Fair Value	132,214	148,635
Total held-to-maturity securities, One Year or less, Fair value	2,715
Total held-to-maturity securities, One to Five Years	10,594
Total held-to-maturity securities, Five to Ten years	115,417
Total held-to-maturity securities, After Ten Years	364,044
Total held-to-maturity securities, Mortgage-backed and Equity
Held-to-maturity securities, Fair value	492,770	479,699
Corporate Debt Securities [Member]
Schedule Of Available For Sale Or Held To Maturity Securities [Line Items]
Total available-for-sale securities, One Year or less	6,141
Total available-for-sale securities, One to Five Years	16,137
Total available-for-sale securities, Five to Ten Years	12,651
Total available-for-sale securities, After Ten Years	7,679
Total available-for-sale securities, Mortgage-backed and Equity
Available-for-sale, Estimated Fair Value	42,608	32,685
Total held-to-maturity securities, One Year or less, Fair value
Total held-to-maturity securities, One to Five Years
Total held-to-maturity securities, Five to Ten years	2,553
Total held-to-maturity securities, After Ten Years
Total held-to-maturity securities, Mortgage-backed and Equity
Held-to-maturity securities, Fair value	2,553
Other Government Agencies [Member]
Schedule Of Available For Sale Or Held To Maturity Securities [Line Items]
Total available-for-sale securities, One Year or less
Total available-for-sale securities, One to Five Years	17,755
Total available-for-sale securities, Five to Ten Years	39,310
Total available-for-sale securities, After Ten Years	13,768
Total available-for-sale securities, Mortgage-backed and Equity
Available-for-sale, Estimated Fair Value	70,833	96,612
Equity Securities [Member]
Schedule Of Available For Sale Or Held To Maturity Securities [Line Items]
Total available-for-sale securities, One Year or less
Total available-for-sale securities, One to Five Years
Total available-for-sale securities, Five to Ten Years
Total available-for-sale securities, After Ten Years
Total available-for-sale securities, Mortgage-backed and Equity	11,107
Available-for-sale, Estimated Fair Value	$ 11,107	$ 11,102