Investment Securities - Summary of Securities Available for Sale in an Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	$ 1,460,814	$ 897,752
Less than 12 months, Unrealized Losses	30,595	7,828
12 months or longer, Fair Value	346,752	369,074
12 months or longer, Unrealized Losses	14,966	13,024
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	116,482	36,678
Less than 12 months, Unrealized Losses	964	230
12 months or longer, Fair Value	22,771	22,920
12 months or longer, Unrealized Losses	97	132
State and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	137,435	82,896
Less than 12 months, Unrealized Losses	1,710	566
12 months or longer, Fair Value	57,150	59,432
12 months or longer, Unrealized Losses	3,524	1,863
Residential Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	664,630	460,414
Less than 12 months, Unrealized Losses	16,051	4,621
12 months or longer, Fair Value	190,576	182,482
12 months or longer, Unrealized Losses	8,511	4,739
Residential Mortgage-Backed Securities, Non-agency [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	0	0
Less than 12 months, Unrealized Losses	0	0
12 months or longer, Fair Value	0	0
12 months or longer, Unrealized Losses	0	0
Commercial Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	415,526	282,858
Less than 12 months, Unrealized Losses	10,289	2,386
12 months or longer, Fair Value	69,225	70,763
12 months or longer, Unrealized Losses	1,647	923
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	75,437	27,931
Less than 12 months, Unrealized Losses	566	7
12 months or longer, Fair Value	0	0
12 months or longer, Unrealized Losses	0	0
Trust Preferred Collateralized Debt Obligations [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	0	0
Less than 12 months, Unrealized Losses	0	0
12 months or longer, Fair Value	2,050	28,629
12 months or longer, Unrealized Losses	450	4,129
Single Issue Trust Preferred Securities [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	0	0
Less than 12 months, Unrealized Losses	0	0
12 months or longer, Fair Value	4,980	4,485
12 months or longer, Unrealized Losses	737	1,225
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value	51,304	6,975
Less than 12 months, Unrealized Losses	1,015	18
12 months or longer, Fair Value	0	0
12 months or longer, Unrealized Losses	$ 0	0
Marketable Equity Securities [Member]
Schedule of Investments [Line Items]
Less than 12 months, Fair Value		0
Less than 12 months, Unrealized Losses		0
12 months or longer, Fair Value		363
12 months or longer, Unrealized Losses		$ 13