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, 2013	Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]
Held to Maturity, Due in one year or less, Amortized Cost	$ 30,674
Held to Maturity, Due after one year through five years, Amortized Cost	381,844
Held to Maturity, Due after five years through ten years, Amortized Cost	178,783
Held to Maturity, Due after ten years, Amortized Cost	2,554,977
Held to Maturity, Residential mortgage-backed securities, Amortized Cost	9,868
Held to Maturity, Equity securities, Amortized Cost	0
Held to Maturity, Amortized Cost	3,156,146	2,956,381
Held to Maturity, Due in one year or less, Estimated Fair Value	31,223
Held to Maturity, Due after one year through five years, Estimated Fair Value	409,840
Held to Maturity, Due after five years through ten years, Estimated Fair Value	175,708
Held to Maturity, Due after ten years, Estimated Fair Value	2,426,229
Held to Maturity, Residential mortgage-backed securities, Estimated Fair Value	9,912
Held to Maturity, Equity securities, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	3,052,912	2,996,944
Available for Sale, Due in one year or less, Amortized Cost	1,014,141
Available for Sale, Due after one year through five years, Amortized Cost	1,573,827
Available for Sale, Due after five years through ten years, Amortized Cost	669,101
Available for Sale, Due after ten years, Amortized Cost	331,509
Available For Sale, Residential mortgage-backed securities, Amortized Cost	1,828,500
Available for Sale, Equity securities, Amortized Cost	35,908
Available for Sale, Amortized Cost	5,452,986	5,990,577
Available for Sale, Due in one year or less, Estimated Fair Value	1,015,777
Available for Sale, Due after one year through five years, Estimated Fair Value	1,597,207
Available for Sale, Due after five years through ten years, Estimated Fair Value	670,434
Available for Sale, Due after ten years, Estimated Fair Value	344,655
Available for Sale, Residential mortgage-backed securities, Estimated Fair Value	1,905,810
Available for Sale, Equity securities, Estimated Fair Value	35,908
Available for Sale, Estimated Fair Value	$ 5,569,791	$ 6,203,299