Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) segment subsidiary	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Property, Plant and Equipment
Loans and Leases Receivable, Past Due Status, Policy [Policy Text Block]	ninety days
Cash and Cash Equivalents	Cash and Cash Equivalents. For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold for one day periods and interest bearing deposits in banks with original maturities of less than three months.
Finite-Lived Intangible Asset, Useful Life	10 years
Real Estate Acquired Through Foreclosure, Valuation Adjustments	$ 900,000	$ 100,000
Number of significant subsidiaries | subsidiary	1
Cash on deposit with Federal Reserve Bank	$ 769,300,000	574,400,000
Compensating balances with Federal Reserve Bank	46,300,000	28,100,000
Deferred compensation plan assets	18,200,000	12,100,000
Deferred compensation plan liabilities	18,200,000	12,100,000
Impairment losses	0	0
Write-downs of OREO	(900,000)	(100,000)	$ (400,000)
Carrying value of foreclosed OREO	2,300,000	2,000,000.0
Federal Reserve Bank stock	42,800,000	37,500,000
Federal Home Loan Bank stock	10,700,000	10,700,000
Income tax examination, penalties	$ 0	0		$ 0
Number of operating segments | segment	1
Advertising expense	$ 4,200,000	3,200,000	3,500,000
Stock compensation expense	8,000,000.0	5,600,000	3,800,000
Stock-based compensation expense		(64,100,000)	(48,600,000)
Mortgage Loans in Process of Foreclosure, Amount	$ 1,400,000
Benefit of stock-based compensation plans			(2,600,000)
Short-term Leases [Policy Text Block]	12 months
Buildings and improvements | Minimum
Property, Plant and Equipment
Estimated useful lives	5 years
Buildings and improvements | Maximum [Member]
Property, Plant and Equipment
Estimated useful lives	45 years
Furniture and equipment | Minimum
Property, Plant and Equipment
Estimated useful lives	4 years
Furniture and equipment | Maximum [Member]
Property, Plant and Equipment
Estimated useful lives	15 years
Restricted Stock [Member]
Property, Plant and Equipment
Stock compensation expense		5,600,000	3,300,000
Tax benefits from stock-based compensation	$ 400,000	200,000	600,000
Stock-based compensation expense		0
Common Stock Including Additional Paid in Capital [Member]
Property, Plant and Equipment
Stock compensation expense	8,000,000.0	5,600,000	3,800,000
Stock-based compensation expense			$ 0
Benefit of stock-based compensation plans	$ (1,200,000)	$ (1,100,000)