Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 168,070
Held to Maturity, Due after one year through five years, Amortized Cost	463,083
Held to Maturity, Due after five years through ten years, Amortized Cost	155,173
Held to Maturity, Due after ten years, Amortized Cost	2,270,211
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	9,330
Held to Maturity, Equity securities, Amortized Cost	0
Held to Maturity, Amortized Cost	3,065,867	3,139,748
Held to Maturity, Due in one year or less, Estimated Fair Value	170,552
Held to Maturity, Due after one year through five years, Estimated Fair Value	489,871
Held to Maturity, Due after five years through ten years, Estimated Fair Value	153,605
Held to Maturity, Due after ten years, Estimated Fair Value	2,248,895
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	9,421
Held to Maturity, Equity securities, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	3,072,344	3,029,663
Available for Sale, Due in one year or less, Amortized Cost	803,553
Available for Sale, Due after one year through five years, Amortized Cost	1,537,991
Available for Sale, Due after five years through ten years, Amortized Cost	1,266,080
Available for Sale, Due after ten years, Amortized Cost	995,435
Available For Sale, Residential mortgage-backed securities, Amortized Cost	1,514,030
Available for Sale, Equity securities, Amortized Cost	43,012
Available for Sale, Amortized Cost	6,160,101	5,799,135
Available for Sale, Due in one year or less, Estimated Fair Value	809,751
Available for Sale, Due after one year through five years, Estimated Fair Value	1,551,000
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,291,383
Available for Sale, Due after ten years, Estimated Fair Value	1,037,236
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	1,590,561
Available for Sale, Equity securities, Estimated Fair Value	43,012
Available for Sale, Estimated Fair Value	$ 6,322,943	$ 5,895,436