Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) shares in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2017	Jun. 30, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2018
Summary Of Significant Accounting Policies [Line Items]
Percentage of preferred stock issued by each REIT - less than			20.00%
Reserve balance in cash or deposits			$ 122,000,000	$ 113,800,000
Allowance for loan losses			120,856,000	114,419,000	$ 106,178,000
FDIC loss-share receivable			6,300,000	7,200,000
OREO and other repossessed assets total			9,800,000	10,200,000
Foreclosed assets acquired in FDIC-assisted transactions				588,000
Foreclosed residential real estate properties			7,300,000	1,600,000
Properties for which formal foreclosure proceedings are in process			$ 3,800,000	$ 7,100,000
Anti-dilutive common stock options and warrants (in shares)			3.1	4.0	4.7
Reclassification due to the adoption of ASU No. 2018-02			$ 0
Noncumulative Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Stock issued (in shares)	4.0	4.6
Minimum
Summary Of Significant Accounting Policies [Line Items]
Premises and equipment, useful life			3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Premises and equipment, useful life			40 years
Commercial loans | Minimum
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be full or partial charged-off			90 days
Commercial loans | Maximum
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be full or partial charged-off			120 days
Residential loans and home equity loans
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be full or partial charged-off			120 days
Automobile
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be fully charged-off			120 days
Unsecured consumer loans
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be fully charged-off			150 days
Commercial and industrial
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses			$ 57,232,000	$ 50,820,000	$ 48,767,000
Loans evaluated for impairment			250,000
PCI Loans
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses			0	0
Trust preferred securities
Summary Of Significant Accounting Policies [Line Items]
Other-than-temporary impairment			0	0	$ 0
PCI Loans
Summary Of Significant Accounting Policies [Line Items]
Covered loans			38,700,000	$ 70,400,000
Accounting Standards Update 2016-16 | Forecast
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect adjustment reducing retained earnings						$ 15,400,000
Retained Earnings
Summary Of Significant Accounting Policies [Line Items]
Reclassification due to the adoption of ASU No. 2018-02			$ 7,927,000