Lease Obligations - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2016 USD ($) branch	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)		Jan. 01, 2018 USD ($)	[2]
Leases [Abstract]
Operating lease, weighted average remaining lease term			11 years 3 months 5 days
Operating lease, weighted average discount rate (as a percent)			3.34%
Operating lease, right-of-use asset			$ 140,700	$ 143,600
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
Adjustments to apply recent accounting pronouncements				47,257	[1]	$ 0
Retained Earnings
Lessee, Lease, Description [Line Items]
Adjustments to apply recent accounting pronouncements				$ 47,257	[1]	$ 6,689

[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. For further discussion of this guidance, see Note 2, "Recent Accounting Pronouncements."

[2] As a result of accounting guidance adopted in the first quarter of 2018, certain reclassifications were made from accumulated other comprehensive loss to retained earnings as of January 1, 2018.