Long-Term Debt - Additional Information (Details)		1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014 USD ($) quarter	Sep. 30, 2021	Dec. 31, 2017 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 05, 2018 USD ($)
Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes
Debt Instrument [Line Items]
Debt instrument, face amount	$ 1,300,000,000				$ 1,300,000,000
Senior note interest rate	4.277%				4.227%
Quarterly principal payment			$ 3,250,000		$ 3,250,000
Number of quarters used in leverage ratio calculation | quarter	4
Deferred finance costs	$ 24,300,000
Amortization of financing costs					$ 3,400,000	$ 3,400,000	$ 3,200,000
Ratio of indebtedness to net capital					4.90
Debt service coverage ratio					4.58
Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes | Maximum
Debt Instrument [Line Items]
Ratio of indebtedness to net capital					5.25
Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes | Forecast
Debt Instrument [Line Items]
Additional interest on fixed rate		5.00%
Debt instrument, term		10 years
Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes | Forecast | Ten Year United States Treasury Bill Rate
Debt Instrument [Line Items]
Basis spread on variable rate		2.15%
Series 2014-1 Variable Funding Senior Notes Class A-1
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 100,000,000
Repayments of debt				$ 20,000,000	$ 30,000,000
Line of credit facility, amount outstanding					$ 30,000,000
Debt, weighted average interest rate					4.50%
Deferred finance costs					$ 2,500,000
Unamortized debt issuance costs					900,000
Costs incurred					$ 1,600,000
Series 2014-1 Variable Funding Senior Notes Class A-1 | Minimum
Debt Instrument [Line Items]
Line of credit facility, commitment fee percentage					0.50%
Series 2014-1 Variable Funding Senior Notes Class A-1 | Maximum
Debt Instrument [Line Items]
Line of credit facility, commitment fee percentage					1.00%
Two Thousand Eighteen Variable Funding Notes
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity					$ 45.0
Debt instrument, term					3 years
Upfront fee unused capacity, percentage					1.00%
Repayments of debt					$ 20.0
Line of credit facility, amount outstanding					$ 25.0
Debt, weighted average interest rate					4.90%
Line of credit facility, current borrowing capacity					$ 196.9
Deferred finance costs					$ 3,700,000
Line of credit facility, commitment fee percentage					0.50%
Unamortized debt issuance costs					$ 3,300,000
First Trigger | Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes
Debt Instrument [Line Items]
Debt service coverage ratio, event trigger threshold					1.75
Debt instrument, restrictive covenant, restriction on net cash flow					50.00%
Second Trigger | Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes
Debt Instrument [Line Items]
Debt service coverage ratio, event trigger threshold					1.5
Debt instrument, restrictive covenant, restriction on net cash flow					100.00%
Third Trigger | Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes
Debt Instrument [Line Items]
Debt service coverage ratio, event trigger threshold					1.3
Fourth Trigger | Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes
Debt Instrument [Line Items]
Debt service coverage ratio, event trigger threshold					1.2
Fifth Trigger | Series 2014-1 Class A-2 4.227% Fixed Rate Senior Secured Notes
Debt Instrument [Line Items]
Debt service coverage ratio, event trigger threshold					1.1
Revolving Credit Facility | Two Thousand Eighteen Variable Funding Notes
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity								$ 225,000,000
Revolving Credit Facility | Two Thousand Eighteen Variable Funding Notes | Eurodollar
Debt Instrument [Line Items]
Basis spread on variable rate					2.15%
Letter of Credit | Two Thousand Eighteen Variable Funding Notes
Debt Instrument [Line Items]
Debt instrument, collateral amount					$ 3,100,000
Letter of Credit | Two Thousand Eighteen Variable Funding Notes | Eurodollar
Debt Instrument [Line Items]
Basis spread on variable rate					1.00%
Letter of Credit | Two Thousand Eighteen Variable Funding Notes | Base Rate
Debt Instrument [Line Items]
Basis spread on variable rate					1.15%
Letter of Credit | Two Thousand Eighteen Variable Funding Notes | Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate					0.50%