Summary of Significant Accounting Policies (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016	Nov. 17, 2016
Basis Of Presentation [Line Items]
Cash and cash equivalents		$ 8,600,000		$ 130,000,000
Interest earning deposits with banks		5,900,000		5,600,000
Federal funds sold		43,800,000		56,100,000
Loans, net of deferred fees and costs		24,538,196,000		23,604,633,000
Federal Reserve Bank stock held		95,600,000		98,600,000	$ 97,300,000
Federal Home Loan Bank stock held		63,900,000		71,300,000
Tax Cuts and Jobs Act of 2017, income tax expense		47,200,000
Share-based compensation, excess tax benefit, amount		5,000,000
Increase in retained earnings		$ 7,800,000
Forecast
Basis Of Presentation [Line Items]
Increase in retained earnings	$ 7,800,000
Increase in regulatory capital	7,800,000
Accounting Standards Update 2016-02 | Forecast
Basis Of Presentation [Line Items]
Operating lease commitments			$ 230,000,000
Operating lease, right-of-use asset			$ 230,000,000
Accounting Standards Update 2016-09 | Retained Earnings | Forecast
Basis Of Presentation [Line Items]
Cumulative effect of new accounting principle in period of adoption	$ 650,000
Non-employee Director
Basis Of Presentation [Line Items]
Vesting period		3 years
Non-employee
Basis Of Presentation [Line Items]
Vesting period		3 years
DRR Methodology
Basis Of Presentation [Line Items]
Loans, net of deferred fees and costs		$ 7,600,000,000		0
Maximum
Basis Of Presentation [Line Items]
Retail loans and commercial loans, evaluated for impairment		$ 1,000,000
Useful life		10 years
Minimum
Basis Of Presentation [Line Items]
Useful life		8 years
Cash And Cash Equivalents
Basis Of Presentation [Line Items]
Total deposits		$ 0		$ 533,000