Borrowed Funds - Additional Information (Detail) - USD ($)	1 Months Ended			12 Months Ended
	Aug. 31, 2016	Jun. 30, 2015	Sep. 30, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Weighted average interest rate for short-term borrowings				1.05%	0.65%
Prepaid long-term FHLB advances					$ 355,000,000	$ 625,000,000
Prepaid long-term FHLB stock					$ 50,000,000	$ 220,000,000
Callable FHLB advances interest rate					3.69%	3.72%
Callable FHLB advances interest rate period					5 years
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				$ 0	315,000	$ 51,129,000
Net carrying value of subordinated debentures				136,000,000	137,700,000
Fair value of securities				$ 1,800,000,000	$ 1,500,000,000
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				3.37%	2.41%
Subordinated notes
Debt Instrument [Line Items]
Subordinated notes		$ 100,000,000		$ 99,200,000	$ 99,000,000
Subordinated notes, interest rate		4.55%
FHLB
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings				2.52%	3.37%
Long-term borrowings
Debt Instrument [Line Items]
Subordinated notes				$ 235,153,000	$ 236,731,000
Subordinated debentures
Debt Instrument [Line Items]
Subordinated notes			$ 125,000,000
Subordinated notes, interest rate			5.125%
Subordinated notes due September 2023 | Subordinated debentures
Debt Instrument [Line Items]
Effective interest rate				3.32%
CNL Bancshares, Inc.
Debt Instrument [Line Items]
Prepaid long-term FHLB advances					$ 87,000,000