Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Within one year, Amortized Cost	$ 20,936
After one but within five years, Amortized Cost	680,506
After five but within ten years, Amortized Cost	673,002
More than ten years, Amortized Cost	268
Total, Amortized Cost	1,374,712	275,507
Within one year, Fair Value	21,053
After one but within five years, Fair Value	680,292
After five but within ten years, Fair Value	669,498
More than ten years, Fair Value	275
Estimated Fair Value	$ 1,371,118	$ 281,924