Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,565,191	$ 1,438,903
Gross Unrealized Gains	25,401	9,781
Gross Unrealized Losses	1,409	9,724
Estimated Fair Value	1,589,183	1,438,960
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	165,176	186,734
Gross Unrealized Gains	2,577	2,234
Gross Unrealized Losses	16	141
Estimated Fair Value	167,737	188,827
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	20,926
Gross Unrealized Gains	69
Estimated Fair Value	20,995
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,379,089	1,252,169
Gross Unrealized Gains	22,755	7,547
Gross Unrealized Losses	1,393	9,583
Estimated Fair Value	$ 1,400,451	$ 1,250,133