Lease Obligations - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($) branch	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)		Dec. 31, 2016 USD ($) branch	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)		Jun. 30, 2019 USD ($)
Leases [Abstract]
Operating lease, weighted average remaining lease term	10 years 5 months 4 days		10 years 5 months 4 days
Operating lease, weighted average discount rate (as a percent)	2.96%		2.96%
Operating lease, right-of-use asset	$ 127,200		$ 127,200
Lessee, Lease, Description [Line Items]
Retail branch network | branch	17
Retail branch network percent	15.00%
Optimization costs	$ 18,376	$ 0	18,376	$ 0
Right-of-use asset	9,100
Impairment charge	8,900
Sale and leaseback transaction, number of branches sold | branch							55
Sale and leaseback transaction, pre-tax gain					$ 65,500		$ 88,000
Sale and leaseback transaction related expenses							$ 5,500
Cumulative effect adjustment	2,664,171	2,339,599	2,664,171	2,339,599	2,054,998			$ 2,656,211	$ 2,370,793		$ 2,300,573
Adjustment to apply recent accounting pronouncements
Lessee, Lease, Description [Line Items]
Cumulative effect adjustment					47,257	[1]			(26,821)	[2]
Retained Earnings
Lessee, Lease, Description [Line Items]
Cumulative effect adjustment	$ 1,366,986	$ 1,343,895	$ 1,366,986	$ 1,343,895	1,192,767			$ 1,359,407	1,380,612		$ 1,304,756
Retained Earnings | Adjustment to apply recent accounting pronouncements
Lessee, Lease, Description [Line Items]
Cumulative effect adjustment					$ 47,257	[1]			$ (26,821)	[2]

[1] As a result of accounting guidance adopted in the first quarter of 2019, the remaining deferred gain on a sale-leaseback transaction was recognized as a cumulative-effect adjustment to retained earnings as of January 1, 2019.

[2] As a result of accounting guidance adopted in the first quarter of 2020, a portion of the increase in allowance for credit losses, net of tax, was recognized as a cumulative-effect adjustment to retained earnings as of January 1, 2020. For further discussion of this guidance, see Note 2, "Recent Accounting Pronouncements and Other Guidance."