Summary of Significant Accounting Policies - Additional Information (Detail) shares in Millions	1 Months Ended		12 Months Ended
	Aug. 31, 2017 shares	Jun. 30, 2015 shares	Dec. 31, 2018 USD ($) shareholder shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of shareholders needed to qualify as a REIT | shareholder			100
Percentage of preferred stock issued by each REIT - less than			20.00%
Reserve balance in cash or deposits			$ 120,700,000	$ 122,000,000
Allowance for loan losses			151,859,000	120,856,000	$ 114,419,000
OREO and other repossessed assets total			9,500,000	9,800,000
Foreclosed residential real estate properties			852,000	7,300,000
Properties for which formal foreclosure proceedings are in process			$ 1,800,000	$ 3,800,000
Anti-dilutive common stock options and warrants (in shares) | shares			2.1	3.1	4.0
Risk-based capital actual, ratio			11.34%	12.61%
Tier 1 risk-based capital actual, ratio			9.30%	10.41%
Noncumulative Preferred Stock
Summary Of Significant Accounting Policies [Line Items]
Stock issued (in shares) | 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 and industrial
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses			$ 90,956,000	$ 57,232,000	$ 50,820,000
Loans evaluated for impairment			250,000
PCI Loans
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses			0	0
PCI Loans
Summary Of Significant Accounting Policies [Line Items]
Covered loans			27,600,000	38,700,000
ASU 2016-16
Summary Of Significant Accounting Policies [Line Items]
Reclassification due to the adoption of an ASU							$ (17,611,000)
ASU 2016-02 | Forecast
Summary Of Significant Accounting Policies [Line Items]
Right-of-use asset						$ 216,000,000
Lease obligations						$ 241,000,000
ASU 2016-02 | Forecast | Minimum
Summary Of Significant Accounting Policies [Line Items]
Risk-based capital actual, ratio						10.00%
Tier 1 risk-based capital actual, ratio						7.00%
ASU 2016-02 | Forecast | Maximum
Summary Of Significant Accounting Policies [Line Items]
Risk-based capital actual, ratio						12.00%
Tier 1 risk-based capital actual, ratio						9.00%
Retained Earnings | ASU 2016-16
Summary Of Significant Accounting Policies [Line Items]
Reclassification due to the adoption of an ASU							$ (17,611,000)
Retained Earnings | ASU 2016-02 | Forecast
Summary Of Significant Accounting Policies [Line Items]
Reclassification due to the adoption of an ASU						$ 6,200,000
Trust preferred securities
Summary Of Significant Accounting Policies [Line Items]
Other-than-temporary impairment			$ 0	$ 0	$ 0
Commitments under commercial loans and lines of credit | Minimum
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be full or partial charged-off			90 days
Commitments under commercial loans and lines of credit | 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 mortgage and home equity
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 loans at banks
Summary Of Significant Accounting Policies [Line Items]
Number of days past due of loans to be fully charged-off			150 days