Debt - Schedule of Long-term Debt Outstanding (Details) - USD ($)	Jun. 30, 2017	Jun. 30, 2016
Revolving Credit Agreement
Total	$ 605,000,000	$ 611,300,000
Less: amounts due within one year	(55,000,000)	0
Long-term debt, net of current portion	550,000,000	611,300,000
Medium-term notes, Series B at 6.97% to 7.10% due from April 2018 to May 2018 (face value of $55.0 million at June 30, 2017 and 2016)
Revolving Credit Agreement
Face amount	55,000,000.0	55,000,000.0
Total	$ 55,000,000	55,000,000
Medium-term notes, Series B at 6.97% to 7.10% due from April 2018 to May 2018 (face value of $55.0 million at June 30, 2017 and 2016) | 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 $55.0 million at June 30, 2017 and 2016) | 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, 2017 and 2016)
Revolving Credit Agreement
Interest rate	5.20%
Face amount	$ 250,000,000.0	250,000,000.0
Total	$ 251,200,000	257,800,000
Senior unsecured notes, 4.45% due March 2023 (face value of $300.0 million at June 30, 2017 and 2016)
Revolving Credit Agreement
Interest rate	4.45%
Face amount	$ 300,000,000.0	300,000,000.0
Total	$ 298,800,000	$ 298,500,000