Investment Securities Held-to-Maturity - Summary of Held-to-Maturity Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 1,722,875	$ 1,653,986
Gross Unrealized Gains	6,353	6,014
Gross Unrealized Losses	22,388	24,192
Estimated Fair Value	1,706,840	1,635,808
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	134,378	148,326
Gross Unrealized Gains	931	1,066
Gross Unrealized Losses	287	527
Estimated Fair Value	135,022	148,865
SBA Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	54,427	46,140
Gross Unrealized Gains	3
Gross Unrealized Losses	779	1,088
Estimated Fair Value	53,651	45,052
U.S. Government Agency and Sponsored Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,534,070	1,459,520
Gross Unrealized Gains	5,419	4,948
Gross Unrealized Losses	21,322	22,577
Estimated Fair Value	$ 1,518,167	$ 1,441,891