Lease Obligations - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) branch
Leases [Abstract]
Operating lease, weighted average remaining lease term	9 years 2 months 12 days		9 years 2 months 12 days
Operating lease, weighted average discount rate (as a percent)	2.71%		2.71%
Operating lease, right-of-use asset	$ 140,200		$ 140,200
Operating lease, right-of-use asset, statement of financial position	Accrued interest receivable and other assets		Accrued interest receivable and other assets
Retail branch network | branch					17
Retail branch network percent					15.00%
Optimization costs	$ 0	$ 18,376	$ 1,556	$ 18,376	$ 19,900
Right-of-use asset					9,100
Impairment charge					$ 8,900