Investment Securities Held-to-Maturity - Company's Securities Held-to-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Within one year, Amortized Cost	$ 3,842
After one but within five years, Amortized Cost	1,419,300
After five but within ten years, Amortized Cost	346,515
More than ten years, Amortized Cost	3,950
Amortized Cost	1,773,607	1,406,792
Within one year, Fair Value	3,908
After one but within five years, Fair Value	1,434,605
After five but within ten years, Fair Value	354,165
More than ten years, Fair Value	4,010
Estimated Fair Value	$ 1,796,688	$ 1,413,603