Investment Securities - Amortized Cost and Fair Value of Debt Securities by Contractual Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013
Investments Debt And Equity Securities [Abstract]
Due in one year or less, Amortized Cost	$ 164,603
Due after one year through five years, Amortized Cost	1,675,196
Due after five years through ten years, Amortized Cost	529,570
Due after ten years, Amortized Cost	55,285
Total Available-for-sale Securities, Debt Maturities, Amortized Cost Basis	2,424,654
Due in one year or less, Fair Value	166,925
Due after one year through five years, Fair Value	1,694,138
Due after five years through ten years, Fair Value	515,942
Due after ten years, Fair Value	54,576
Available-for-sale Securities, Debt Maturities fair value	$ 2,431,581
Due in one year or less, Weighted-Average Yield	1.73%
Due after one year through five years, Weighted-Average Yield	2.14%
Due after five years through ten years, Weighted-Average Yield	2.37%
Due after ten years, Weighted-Average Yield	3.49%
Total available for sale Securities Debt Maturities, Weighted-Average Yield	2.20%