Summary of Significant Accounting Policies (Details)			12 Months Ended
	Dec. 22, 2017 Rate	Dec. 18, 2017 USD ($)	Dec. 31, 2017 USD ($) branch	Dec. 31, 2017 USD ($) vote branch	Dec. 31, 2017 USD ($) basis_points branch	Dec. 31, 2017 USD ($) branch Rate	Dec. 31, 2017 USD ($) branch	Dec. 31, 2017 USD ($) branch	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 01, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Federal statutory rate	21.00%					35.00%		35.00%
Total purchase price		$ 113,600,000
Net impact from FDIC shared-loss termination							$ (45,000)		$ 16,559,000	$ 0
Gain on acquisitions							134,745,000		5,831,000	$ 42,930,000
Nonmarketable Securities and Federal Home Loan Bank Stock			$ 53,000,000	$ 53,000,000	$ 53,000,000	$ 53,000,000	$ 53,000,000	$ 53,000,000	43,800,000
Period fair values are subject to refinement			1 year
Number of branches | branch			545	545	545	545	545	545
Baseline for measurement			$ 500,000	$ 500,000	$ 500,000	$ 500,000	$ 500,000	$ 500,000
Short-term borrowings			693,807,000	693,807,000	693,807,000	693,807,000	693,807,000	693,807,000	603,487,000
Amortization Method Qualified Affordable Housing Project Investments			$ 128,000,000	$ 128,000,000	$ 128,000,000	$ 128,000,000	128,000,000	$ 128,000,000	$ 109,800,000
Class A Common Stock
Property, Plant and Equipment [Line Items]
Votes per share of common stock | vote				1
Class B Common Stock
Property, Plant and Equipment [Line Items]
Votes per share of common stock | vote				16
Minimum
Property, Plant and Equipment [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification							65,000,000
Maximum
Property, Plant and Equipment [Line Items]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification							$ 85,000,000
Premises | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of asset			3 years
Premises | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of asset			40 years
Furniture, Software, and Equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life of asset			3 years
Furniture, Software, and Equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life of asset			10 years
Income tax ASU cumulative effect adjustment [Member] | Subsequent Event
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption											$ 27,200,000
Financial Instruments ASU cumulative effect adjustment [Member] | Subsequent Event
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption											$ 18,700,000
Capital basis points | Minimum
Property, Plant and Equipment [Line Items]
New Accounting Pronouncement or Change in Accounting Principle Effect of Adoption Capital Quantification | basis_points					4
Capital basis points | Maximum
Property, Plant and Equipment [Line Items]
New Accounting Pronouncement or Change in Accounting Principle Effect of Adoption Capital Quantification | basis_points					6