Fair Value of Financial Instruments - Carrying Value and Fair Value, Senior Notes and Finance Receivables (Details) - Level 2 - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Carrying value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance receivables	$ 180,856	$ 105,200
Carrying value | Senior Unsecured Notes Effective September 2018 | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	590,518	342,869
Fair value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Finance receivables	191,561	109,703
Fair value | Senior Unsecured Notes Effective September 2018 | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior Notes	$ 608,056	$ 319,375