Debt - Schedule of Long-term Debt Outstanding (Details) - USD ($)	Jun. 30, 2018	Jun. 30, 2017
Revolving Credit Agreement
Total	$ 545,700,000	$ 605,000,000
Less: amounts due within one year	0	(55,000,000)
Long-term debt, net of current portion	545,700,000	550,000,000
Medium-term notes, Series B at 6.97% to 7.10% due from April 2018 to May 2018 (face value of $0.0 and $55.0 million at June 30, 2018 and 2017, respectively)
Revolving Credit Agreement
Face amount	0	55,000,000.0
Total	$ 0	55,000,000
Medium-term notes, Series B at 6.97% to 7.10% due from April 2018 to May 2018 (face value of $0.0 and $55.0 million at June 30, 2018 and 2017, respectively) \| Minimum
Revolving Credit Agreement
Interest rate	6.97%
Medium-term notes, Series B at 6.97% to 7.10% due from April 2018 to May 2018 (face value of $0.0 and $55.0 million at June 30, 2018 and 2017, respectively) \| Maximum
Revolving Credit Agreement
Interest rate	7.10%
Senior unsecured notes, 5.20% due July 2021 (face value of $250.0 million at June 30, 2018 and 2017)
Revolving Credit Agreement
Interest rate	5.20%
Face amount	$ 250,000,000.0	250,000,000.0
Total	$ 246,600,000	251,200,000
Senior unsecured notes, 4.45% due March 2023 (face value of $300.0 million at June 30, 2018 and 2017)
Revolving Credit Agreement
Interest rate	4.45%
Face amount	$ 300,000,000.0	300,000,000.0
Total	$ 299,100,000	$ 298,800,000

Face (par) amount of debt instrument at time of issuance.

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