Credit Agreement (Details) - USD ($)		1 Months Ended	3 Months Ended
	Feb. 26, 2016	Oct. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019	Aug. 09, 2017
Credit Agreement
Amount outstanding			$ 320,000,000		$ 220,000,000
Proceeds from credit agreement			100,000,000	$ 0
Line of Credit | Credit Agreement
Credit Agreement
Principal amount	$ 250,000,000.0					$ 350,000,000.0
Amount outstanding			$ 320,000,000.0		$ 220,000,000.0
Proceeds from credit agreement		$ 15,000,000.0
Margin rate (as a percent)	0.50%
Maximum leverage ratio			3.0
Minimum interest coverage ratio			3.0
Line of Credit | Credit Agreement | Minimum
Credit Agreement
Commitment fee percentage (as a percent)	0.20%
Line of Credit | Credit Agreement | Maximum
Credit Agreement
Commitment fee percentage (as a percent)	0.30%
Line of Credit | Credit Agreement | Federal Funds Effective Rate
Credit Agreement
Variable rate spread (as a percent)	0.50%
Line of Credit | Credit Agreement | Adjusted LIBOR
Credit Agreement
Variable rate spread (as a percent)	1.00%
Margin rate (as a percent)	1.50%
Outstanding Borrowing 1 [Member] | Line of Credit | Credit Agreement
Credit Agreement
Amount outstanding			$ 80,000,000.0
Effective interest rate (as a percent)			3.54%
Outstanding Borrowing 3 [Member] | Line of Credit | Credit Agreement
Credit Agreement
Amount outstanding			$ 61,000,000.0
Effective interest rate (as a percent)			3.57%
Outstanding Borrowing 4 [Member] | Line of Credit | Credit Agreement
Credit Agreement
Amount outstanding			$ 159,000,000.0
Effective interest rate (as a percent)			3.55%
Outstanding Borrowing 2 [Member] | Line of Credit | Credit Agreement
Credit Agreement
Amount outstanding			$ 20,000,000.0
Effective interest rate (as a percent)			3.56%