Investment Securities - Contractual Maturities of Debt Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
Due in one year, Amortized Cost	$ 65,770
Due after one year through five years, Amortized Cost	65,368
Due after five years through ten years, Amortized Cost	336,469
Due after ten years, Amortized Cost	287,321
Residential mortgage-backed securities, Amortized Cost	965,647
Held to Maturity, Amortized Cost	1,720,575	$ 1,778,316
Due in one year, Fair Value	65,782
Due after one year through five years, Fair Value	69,690
Due after five years through ten years, Fair Value	353,088
Due after ten years, Fair Value	281,187
Residential mortgage-backed securities, Fair Value	969,548
Fair Value	$ 1,739,295	$ 1,815,976