Securities Available-for-Sale - Maturity Distribution of Securities Available-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Within one year, Amortized Cost	$ 55,219
After one but within five years, Amortized Cost	753,219
After five but within ten years, Amortized Cost	747,073
More than 10 years, Amortized Cost	11,478
Non-maturing, Amortized Cost	1,947
Total, Amortized Cost	1,568,936
Within one year, Fair Value	55,325
After one but within five years, Fair Value	755,445
After five but within ten years, Fair Value	728,512
More than 10 years, Fair Value	10,945
Non-maturing, Fair Value	1,989
Total, Fair Value	$ 1,552,216	$ 1,434,801