Credit Agreement (Details)	Feb. 26, 2016 USD ($)	Mar. 31, 2018 USD ($)	Aug. 09, 2017 USD ($)	Jun. 30, 2017 USD ($)
Credit Agreement
Amount outstanding		$ 170,000,000		$ 140,000,000
Line of Credit | Credit Agreement
Credit Agreement
Principal amount	$ 250,000,000.0		$ 350,000,000.0
Amount outstanding		$ 170,000,000		$ 140,000,000
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		$ 159,000,000
Effective interest rate (as a percent)		3.38%
Outstanding Borrowing 2 [Member] | Line of Credit | Credit Agreement
Credit Agreement
Amount outstanding		$ 11,000,000
Effective interest rate (as a percent)		3.22%