Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 81,860
Due after one year through five years, Amortized Cost	71,354
Due after five years through ten years, Amortized Cost	374,915
Due after ten years, Amortized Cost	220,035
Residential mortgage-backed securities, Amortized Cost	889,146
Held to Maturity, Amortized Cost	1,637,310	$ 1,778,316
Due in one year, Fair Value	81,866
Due after one year through five years, Fair Value	75,970
Due after five years through ten years, Fair Value	400,050
Due after ten years, Fair Value	213,289
Residential mortgage-backed securities, Fair Value	899,776
Fair Value	$ 1,670,951	$ 1,815,976