Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 124,693
Held to Maturity, Due after one year through five years, Amortized Cost	498,817
Held to Maturity, Due after five years through ten years, Amortized Cost	186,813
Held to Maturity, Due after ten years, Amortized Cost	2,123,244
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	8,445
Held to Maturity, Equity securities, Amortized Cost	0
Held to Maturity, Amortized Cost	2,942,012	3,139,748
Held to Maturity, Due in one year or less, Estimated Fair Value	127,767
Held to Maturity, Due after one year through five years, Estimated Fair Value	530,551
Held to Maturity, Due after five years through ten years, Estimated Fair Value	186,085
Held to Maturity, Due after ten years, Estimated Fair Value	2,117,890
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	8,492
Held to Maturity, Equity securities, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	2,970,785	3,029,663
Available-for-sale Securities, Due in one year or less, Amortized Cost	787,678
Available for Sale, Due after one year through five years, Amortized Cost	1,792,593
Available for Sale, Due after five years through ten years, Amortized Cost	1,459,094
Available for Sale, Due after ten years, Amortized Cost	1,241,583
Available For Sale, Residential mortgage-backed securities, Amortized Cost	1,415,433
Available for Sale, Equity securities, Amortized Cost	42,998
Available for Sale, Amortized Cost	6,739,379	5,799,135
Available for Sale, Due in one year or less, Estimated Fair Value	791,624
Available for Sale, Due after one year through five years, Estimated Fair Value	1,806,723
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,488,506
Available for Sale, Due after ten years, Estimated Fair Value	1,298,190
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	1,483,611
Available for Sale, Equity securities, Estimated Fair Value	42,998
Available for Sale, Estimated Fair Value	$ 6,911,652	$ 5,895,436