Debt (Senior Credit Facility) (Details)		9 Months Ended		12 Months Ended
	Oct. 15, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Contributions from member		$ 13,588,000	$ 10,825,000
Term Loan | Term Loan B
Line of Credit Facility [Line Items]
Line of credit availability		240,700,000
Line of credit, outstanding balance		235,800,000
Senior credit facility
Line of Credit Facility [Line Items]
Remainder amount available for borrowing		$ 271,000,000.0
Senior credit facility | Maximum
Line of Credit Facility [Line Items]
Leverage ratio		6.50
Secured leverage ratio		4.00
Senior credit facility | Term Loan B
Line of Credit Facility [Line Items]
Principal payments, a percentage of the original principal amount		0.25
Senior credit facility | Term Loan B | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit availability	$ 400,000,000.0
Senior credit facility | Term Loan B | Base Rate | Subsequent Event
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	alternate base rate
Basis spread on variable rate (as a percent)	1.75%
Senior credit facility | Term Loan B | LIBOR
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
Basis spread on variable rate (as a percent)		2.25%
Senior credit facility | Term Loan B | LIBOR | Subsequent Event
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Basis spread on variable rate (as a percent)	2.75%
Variable rate basis floor (as a percent)	0.75%
Senior credit facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit availability		$ 550,000,000.0
Contributions from member				$ 50,000,000.0
Line of credit, outstanding balance		$ 275,000,000.0
Senior credit facility | Revolving Credit Facility | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit availability	$ 550,000,000
Senior credit facility | Revolving Credit Facility | Base Rate | Subsequent Event
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	alternate base rate
Senior credit facility | Revolving Credit Facility | Base Rate | Minimum | Subsequent Event
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)	0.50%
Senior credit facility | Revolving Credit Facility | Base Rate | Maximum | Subsequent Event
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)	1.75%
Senior credit facility | Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR
Senior credit facility | Revolving Credit Facility | LIBOR | Subsequent Event
Line of Credit Facility [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR
Senior credit facility | Revolving Credit Facility | LIBOR | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)		1.50%
Senior credit facility | Revolving Credit Facility | LIBOR | Minimum | Subsequent Event
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)	1.50%
Senior credit facility | Revolving Credit Facility | LIBOR | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)		2.75%
Senior credit facility | Revolving Credit Facility | LIBOR | Maximum | Subsequent Event
Line of Credit Facility [Line Items]
Basis spread on variable rate (as a percent)	2.75%
Senior credit facility | Letters of Credit
Line of Credit Facility [Line Items]
Line of credit, outstanding balance		$ 4,000,000.0
Senior credit facility | Letters of Credit | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit availability	$ 25,000,000
Senior credit facility | 2024 Senior Notes | Subsequent Event
Line of Credit Facility [Line Items]
Outstanding Principal	$ 325,000,000
Senior credit facility | Interest Period One | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit, interest period	1 month
Senior credit facility | Interest Period Two | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit, interest period	2 months
Senior credit facility | Interest Period Three | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit, interest period	3 months
Senior credit facility | Interest Period Four | Subsequent Event
Line of Credit Facility [Line Items]
Line of credit, interest period	6 months