Loans - Additional Information (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Non-PCI loans net of premiums and deferred loan fees	$ 22,200,000				$ 15,300,000				$ 22,200,000	$ 15,300,000
Loans	18,331,580,000				17,236,103,000				18,331,580,000	17,236,103,000
Loans transferred to loans held for sale									313,201,000	174,501,000	$ 0
Gains on sales of loans, net	$ 6,375,000	$ 5,520,000	$ 4,791,000	$ 4,128,000	12,307,000	$ 4,823,000	$ 3,105,000	$ 1,795,000	20,814,000	22,030,000	4,245,000
Sales of loans									0	0
Net increase (decrease) in expected cash flows									$ 77,265,000	(9,989,000)
Combined loan-to-value ratio home equity loan	80.00%								80.00%
Accrued interest on non-accrual loans									$ 2,500,000	2,100,000	$ 3,500,000
TDRs not reported as non-accrual loans	$ 117,200,000				85,200,000				117,200,000	85,200,000
Non-performing TDRs	27,000,000				10,600,000				27,000,000	10,600,000
Specific reserves for loan losses									8,700,000	4,800,000
Troubled debt restructuring, charge-offs									$ 0	$ 0
Number of days loans placed on non-accrual status									90 days	90 days
PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,387,215,000				1,771,502,000				$ 1,387,215,000	$ 1,771,502,000
Covered loans	38,700,000				70,400,000				38,700,000	70,400,000
PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	1,500,000,000				1,900,000,000				1,500,000,000	1,900,000,000
Credit card
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unsecured loans	8,200,000				7,000,000				8,200,000	7,000,000
Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,859,035,000				2,867,918,000				2,859,035,000	2,867,918,000
Loans transferred to loans held for sale									313,200,000	174,500,000
Gains on sales of loans, net									8,800,000	7,300,000
Residential mortgage | PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	141,291,000				183,723,000				141,291,000	183,723,000
Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,741,425,000				2,638,195,000				2,741,425,000	2,638,195,000
Unsecured loans	401,800,000				455,500,000				401,800,000	455,500,000
Impaired loans	250,000								250,000
Commercial and industrial | PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	192,360,000				281,177,000				192,360,000	281,177,000
Commercial and industrial | Tax Medallion
Accounts, Notes, Loans and Financing Receivable [Line Items]
Taxi medallion loans	137,300,000								137,300,000
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	2,383,238,000				2,185,377,000				2,383,238,000	2,185,377,000
Unsecured loans	18,100,000				20,600,000				18,100,000	20,600,000
Consumer | PCI Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 77,497,000				$ 100,583,000				$ 77,497,000	$ 100,583,000