Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 3,837
After one but within five years, Amortized Cost	1,095,478
After five but within ten years, Amortized Cost	305,690
More than ten years, Amortized Cost	1,787
Total, Amortized Cost	1,406,792	1,487,884
Within one year, Fair Value	3,912
After one but within five years, Fair Value	1,098,957
After five but within ten years, Fair Value	308,892
More than ten years, Fair Value	1,842
Estimated Fair Value	$ 1,413,603	$ 1,464,449