Summary of Significant Accounting Policies - New Accounting Standards (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
New Accounting Pronouncement, Early Adoption [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Finance receivables	$ 6,833,944		$ 7,572,947	$ 7,411,816	$ 7,157,557	$ 7,008,801
Allowance for credit losses on finance receivables	(390,936)		(198,581)	$ (189,885)	$ (192,471)	$ (173,343)
Deferred income taxes	158,538		101,204
Accrued liabilities			582,288
Retained earnings	$ 1,284,823		2,193,997
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncement, Early Adoption [Line Items]
Finance receivables		$ 0
Allowance for credit losses on finance receivables		(100,604)	$ (100,604)
Deferred income taxes		22,484
Accrued liabilities		109
Retained earnings		(78,229)
Cumulative Effect, Period of Adoption, Adjusted Balance
New Accounting Pronouncement, Early Adoption [Line Items]
Finance receivables		7,572,947
Allowance for credit losses on finance receivables		(299,185)
Deferred income taxes		123,688
Accrued liabilities		582,397
Retained earnings		$ 2,115,768