Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 690,637,000		$ 690,637,000		$ 690,637,000
Goodwill impairment	0	$ 0	0	$ 0
Finite-Lived Intangible Assets [Line Items]
Amortization of other intangible assets	2,498,000	2,675,000	$ 7,596,000	8,452,000
Core Deposits
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years			11 years
Other
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years			20 years
Core Deposits and Other
Finite-Lived Intangible Assets [Line Items]
Impairment of core deposits and other intangibles	$ 0	$ 0	$ 0	$ 0