Summary of Significant Accounting Policies - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 USD ($) subsidiary	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Property, Plant and Equipment
Benefit of stock-based compensation plans			$ (1,100,000)
Impairment losses	$ 0	$ 0	0
Number of significant subsidiaries | subsidiary	1
Income tax examination, penalties	$ 0	0	0
Cash on deposit with Federal Reserve Bank	1,989,300,000	769,300,000
Compensating balances with Federal Reserve Bank	0	46,300,000
Federal Reserve Bank stock	42,800,000	42,800,000
Federal Home Loan Bank stock	10,600,000	10,700,000
Advertising expense	2,800,000	4,200,000	$ 3,200,000
Loans and Leases Receivable Disclosure [Line Items]
Accrued interest receivable	51,100,000	46,700,000
Accrued Investment Income Receivable	12,600,000	9,800,000
Hosting Arrangement, Service Contract, Implementation Cost, Expense, Amortization	6,000,000.0	9,200,000
Impairment losses	$ 0	0
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	3 years
Furniture and equipment | Maximum [Member]
Property, Plant and Equipment
Estimated useful lives	15 years
Loans
Loans and Leases Receivable Disclosure [Line Items]
Accrued interest receivable	$ 37,900,000	36,900,000
Common Stock Including Additional Paid in Capital [Member]
Property, Plant and Equipment
Benefit of stock-based compensation plans	$ (400,000)	$ (1,200,000)