Borrowed Funds - Additional Information (Detail) - USD ($)		1 Months Ended			12 Months Ended
	Jan. 01, 2018	Aug. 31, 2016	Jun. 30, 2015	Sep. 30, 2013	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Weighted average interest rate for short-term borrowings						2.45%	1.05%
Prepaid long-term FHLB advances					$ 355,000,000
Prepaid long-term FHLB stock					$ 50,000,000
Callable FHLB advances interest rate					3.69%
FHLB advances					$ 405,000,000
Callable FHLB advances adjusted interest rate					2.51%
Prepayment penalties paid to FHLB					$ 20,000,000
Loss on extinguishment of debt					315,000
Subordinated notes, interest rate	6.25%
Fair value of securities						$ 2,400,000,000	$ 1,900,000,000
Interest rate swaps
Debt Instrument [Line Items]
Notional amount of terminated derivative		$ 125,000,000
Securities sold under agreements to repurchase
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings						3.70%	3.37%
FHLB
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings						3.13%	2.52%
Subordinated notes
Debt Instrument [Line Items]
Subordinated notes			$ 100,000,000
Subordinated notes, interest rate			4.55%
Net carrying value of subordinated debentures						$ 99,300,000	$ 99,200,000
Subordinated notes due September 2023
Debt Instrument [Line Items]
Subordinated notes				$ 125,000,000
Subordinated notes, interest rate				5.125%
Net carrying value of subordinated debentures						134,200,000	$ 135,200,000
Effective interest rate		3.32%
CNL Bancshares, Inc.
Debt Instrument [Line Items]
Prepaid long-term FHLB advances					$ 87,000,000
USAmeriBancorp, Inc.
Debt Instrument [Line Items]
Subordinated notes	$ 60,000,000
Net carrying value of subordinated debentures						$ 61,100,000