Summary of Significant Accounting Policies Narrative (Details)		12 Months Ended
	Jan. 01, 2018 USD ($)	Dec. 31, 2020 USD ($) subsidiarytrusts	Dec. 31, 2018 USD ($) subsidiarytrusts	Mar. 31, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)
Property, Plant and Equipment [Line Items]
Number of banks owned | subsidiarytrusts			3
Days past due for nonaccrual status		90 days
Financing receivable, obtaining certified third-party appraisal for impaired loans, period		12 months
Reclassification of stranded tax effects			$ 0
Subsidiary trusts owned by parent | subsidiarytrusts		3
Extension term of leases		5 years
Increase in ACL		$ 291,940,000	$ 169,410,000			$ 166,209,000	$ 176,084,000
Change in retained earnings		1,120,781,000				1,079,391,000
Foreign currency open position		$ 500,000
Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Increase in ACL						58,348,000
Building and Building Improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		50 years
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		8 years
Equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life		5 years
Consumer Loan
Property, Plant and Equipment [Line Items]
Number of days closed end consumer loans are charged off when they become past due		120 days
Number of days open end consumer loans are charged off when they become past due		180 days
Residential Mortgage
Property, Plant and Equipment [Line Items]
Change in ACL for certain OBS credit exposure		$ 1,100,000				$ 3,200,000
Accounting Standards Update 2018-02
Property, Plant and Equipment [Line Items]
Reclassification of stranded tax effects	$ 7,100,000
Accounting Standards Update 2016-13 | Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Increase in ACL					$ 58,300,000
Change in retained earnings					43,800,000
Change in deferred tax assets					12,400,000
Accounting Standards Update 2016-13 | Residential Mortgage
Property, Plant and Equipment [Line Items]
Change in ACL for certain OBS credit exposure				$ (2,100,000)
Accounting Standards Update 2016-13 | Residential Mortgage | Cumulative Effect, Period of Adoption, Adjustment
Property, Plant and Equipment [Line Items]
Change in ACL for certain OBS credit exposure					$ 2,100,000
Employee Equity Plan
Property, Plant and Equipment [Line Items]
Award vesting period		3 years
Directors' Plan
Property, Plant and Equipment [Line Items]
Award vesting period		1 year
Minimum
Property, Plant and Equipment [Line Items]
Remaining lease term of operating leases		1 year
Maximum
Property, Plant and Equipment [Line Items]
Remaining lease term of operating leases		20 years