Credit Agreement - Narrative (Details)	Feb. 26, 2016 USD ($)	Jun. 30, 2018 USD ($)	Aug. 09, 2017 USD ($)	Jun. 30, 2017 USD ($)
Line of Credit Facility [Line Items]
Borrowings under credit agreement		$ 170,000,000		$ 140,000,000
Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Principal amount	$ 250,000,000.0		$ 350,000,000.0
Margin rate (as a percent)	0.50%
Maximum leverage ratio		3.0
Minimum interest coverage ratio		3.0
Federal Funds Effective Rate | Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Variable rate spread (as a percent)	0.50%
Adjusted LIBOR | Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Variable rate spread (as a percent)	1.00%
Margin rate (as a percent)	1.50%
Minimum | Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Commitment fee percentage (as a percent)	0.20%
Maximum | Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Commitment fee percentage (as a percent)	0.30%
Outstanding Borrowing 1 [Member] | Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Borrowings under credit agreement		$ 159,000,000
Effective interest rate (as a percent)		3.60%
Outstanding Borrowing 2 [Member] | Credit Agreement | Line of Credit
Line of Credit Facility [Line Items]
Borrowings under credit agreement		$ 11,000,000
Effective interest rate (as a percent)		3.55%