Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
	Jan. 01, 2018	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2016
Basis Of Presentation [Line Items]
Cash and cash equivalents				$ 8,600
Federal funds sold			$ 25,600	49,700
Loans, net			$ 25,696,018	24,538,196
Separate account life insurance				15,000
Tax Cuts and Jobs Act of 2017, provisional income tax expense				47,200
Vesting period			3 years
Increase in retained earnings	$ 7,600	$ 7,600		7,600
Cumulative effect of new accounting principle in period of adoption			$ (685)	0		$ 0
Hosting arrangements under service contract, costs capitalized			$ 500
Cumulative-effect adjustment from adoption of ASU 2014-09
Basis Of Presentation [Line Items]
Cumulative effect of new accounting principle in period of adoption				(685)
Cumulative-effect adjustment from adoption of ASU 2014-09 | Retained Earnings
Basis Of Presentation [Line Items]
Cumulative effect of new accounting principle in period of adoption				(685)
Cumulative-effect adjustment from adoption of ASU 2016-01
Basis Of Presentation [Line Items]
Cumulative effect of new accounting principle in period of adoption	3,200			0
Cumulative-effect adjustment from adoption of ASU 2016-01 | Accumulated Other Comprehensive Income (Loss)
Basis Of Presentation [Line Items]
Cumulative effect of new accounting principle in period of adoption	$ 117			117
Cumulative-effect adjustment from adoption of ASU 2016-01 | Retained Earnings
Basis Of Presentation [Line Items]
Cumulative effect of new accounting principle in period of adoption				(117)
Accounting Standards Update 2016-02 | Forecast
Basis Of Presentation [Line Items]
Additional operating liabilities recognized as percent of total assets					1.10%
Sale leaseback gains					$ 3,900
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			$ 7,600,000	$ 7,600,000
Maximum
Basis Of Presentation [Line Items]
Retail loans and commercial loans, evaluated for impairment			$ 1,000
Useful life			10 years
Maximum | Buildings and improvements
Basis Of Presentation [Line Items]
Useful life			40 years
Maximum | Furniture and equipment
Basis Of Presentation [Line Items]
Useful life			10 years
Minimum
Basis Of Presentation [Line Items]
Useful life			8 years
Minimum | Buildings and improvements
Basis Of Presentation [Line Items]
Useful life			10 years
Minimum | Furniture and equipment
Basis Of Presentation [Line Items]
Useful life			3 years