Lease Obligations - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)		Dec. 31, 2016 USD ($) branch	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)		Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)
Leases [Abstract]
Operating lease, weighted average remaining lease term				10 years 8 months 12 days
Operating lease, weighted average discount rate (as a percent)				2.96%
Operating lease, right-of-use asset				$ 141,200
Lessee, Lease, Description [Line Items]
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	2,054,998			2,656,211	$ 2,435,707	$ 2,370,793		$ 2,300,573	$ 2,159,471
Cumulative Effect, Period Of Adoption, Adjustment
Lessee, Lease, Description [Line Items]
Cumulative effect adjustment	47,257	[1]				(26,821)	[2]
Retained Earnings
Lessee, Lease, Description [Line Items]
Cumulative effect adjustment	1,192,767			$ 1,359,407	$ 1,357,395	1,380,612		$ 1,304,756	$ 1,273,245
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment
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."