Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Within one year, Amortized Cost	$ 28,236
After one but within five years, Amortized Cost	173,851
After five but within ten years, Amortized Cost	55,566
More than ten years, Amortized Cost	9,960
Total, Amortized Cost	267,613	275,507
Within one year, Fair Value	28,390
After one but within five years, Fair Value	175,064
After five but within ten years, Fair Value	55,389
More than ten years, Fair Value	9,959
Estimated Fair Value	$ 268,802	$ 281,924