Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill And Other Intangible Assets [Line Items]
Amortization of core deposit and other intangible assets	$ 31,366,000	$ 42,613,000	$ 26,424,000
Minimum [Member]
Goodwill And Other Intangible Assets [Line Items]
Estimated useful lives	5 years
Maximum [Member]
Goodwill And Other Intangible Assets [Line Items]
Estimated useful lives	10 years
Core Deposit and Other Intangible Assets [Member]
Goodwill And Other Intangible Assets [Line Items]
Estimated useful lives	8 years
Amortization of core deposit and other intangible assets	$ 31,366,000	$ 42,613,000	$ 26,424,000
Core Deposit and Other Intangible Assets [Member] | Minimum [Member]
Goodwill And Other Intangible Assets [Line Items]
Estimated useful lives	5 years
Core Deposit and Other Intangible Assets [Member] | Maximum [Member]
Goodwill And Other Intangible Assets [Line Items]
Estimated useful lives	10 years