Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Region	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of Metropolitan Statistical Areas | Region	5
Loan fees net of costs amortized and included in interest income	$ 44,475,000	$ 18,471,000	$ 15,798,000
Minimum days, related to accrual of interest on discontinued commercial and consumer loans	90 days
Maximum days, related to accrual of interest on discontinued commercial and consumer loans	120 days
Sustained period of repayment performance in restructured loans	6 months
Real estate acquired in foreclosure or other settlement of loans	$ 24,348,000	31,510,000
Recorded investment of consumer mortgage loans	873,000	660,000
Advertising expense	4,519,000	3,410,000	4,111,000
Amortization expense on intangible assets	7,772,000	3,944,000	3,420,000
Total goodwill	1,478,380,000	863,767,000
Stock based compensation expense	3,555,000	2,817,000	2,484,000
Increase to net cash from operating activity	253,928,000	170,734,000	174,901,000
Decrease to net cash from financing activities	$ (288,967,000)	375,498,000	114,668,000
ASU 2016-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in income tax expense		1,048,000
Increase to net cash from operating activity		4,008,000	4,008,000
Decrease to net cash from financing activities		$ 1,023,000	$ 1,023,000
Minimum [Member] | Core Deposit Intangible Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets amortization period	1 year
Maximum [Member] | Core Deposit Intangible Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Intangible assets amortization period	7 years
Furniture, Fixtures and Equipment [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Bank premises and equipment, useful life	3 years
Furniture, Fixtures and Equipment [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Bank premises and equipment, useful life	15 years
Buildings and Improvements [Member] | Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Bank premises and equipment, useful life	5 years
Buildings and Improvements [Member] | Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Bank premises and equipment, useful life	40 years