Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Available-for-Sale Securities [Line Items]
Due in one year or less, Amortized Cost	$ 7,960
Due from one year to five years, Amortized Cost	47,037
Due from five to ten years, Amortized Cost	66,573
Due after ten years, Amortized Cost	71,166
Amortized Cost	858,010	$ 533,219	$ 530,456
Due in one year or less,Fair Value	7,999
Due from one year to five years, Fair Value	48,246
Due from five to ten years, Fair Value	67,686
Due after ten years, Fair Value	71,023
Mortgage-backed securities, Fair Value	667,200
Total Fair Value	862,154	541,805	535,630
Mortgage-Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Amortized Cost	665,274	369,581	361,194
Total Fair Value	$ 667,200	$ 374,712	$ 364,447