Investment Securities - Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 226,351
Due after one year through five years, Amortized Cost	1,566,483
Due after five years through ten years, Amortized Cost	492,241
Due after ten years, Amortized Cost	44,817
Total Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	2,329,892
Due in one year or less, Fair Value	228,686
Due after one year through five years, Fair Value	1,613,119
Due after five years through ten years, Fair Value	501,892
Due after ten years, Fair Value	46,976
Available-for-sale Securities, Debt Maturities fair value	$ 2,390,673
Due in one year or less, Weighted-Average Yield	1.83%
Due after one year through five years, Weighted-Average Yield	2.36%
Due after five years through ten years, Weighted-Average Yield	2.54%
Due after ten years, Weighted-Average Yield	3.45%
Total available for sale Securities Debt Maturities, Weighted-Average Yield	2.36%