Debt - Schedule of long-term debt outstanding (Details) - USD ($)	Sep. 30, 2017	Jun. 30, 2017
Debt Instrument [Line Items]
Total	$ 604,800,000	$ 605,000,000
Less: amounts due within one year	55,000,000	55,000,000
Long-term debt, net of current portion	549,800,000	550,000,000
Medium-term notes, Series B 6.97% to 7.10% due April 2018 To May 2018
Debt Instrument [Line Items]
Face amount	55,000,000.0	55,000,000.0
Total	$ 55,000,000	$ 55,000,000
Medium-term notes, Series B 6.97% to 7.10% due April 2018 To May 2018 | Minimum
Debt Instrument [Line Items]
Interest rate	6.97%	6.97%
Medium-term notes, Series B 6.97% to 7.10% due April 2018 To May 2018 | Maximum
Debt Instrument [Line Items]
Interest rate	7.10%	7.10%
Senior unsecured notes, 5.20% due July 2021
Debt Instrument [Line Items]
Interest rate	5.20%	5.20%
Face amount	$ 250,000,000.0	$ 250,000,000.0
Total	$ 250,900,000	$ 251,200,000
4.45% Senior Unsecured Notes due 2023
Debt Instrument [Line Items]
Interest rate	4.45%	4.45%
Face amount	$ 300,000,000.0	$ 300,000,000.0
Total	$ 298,900,000	$ 298,800,000