Debt - Schedule of long-term debt outstanding (Details) - USD ($)	Sep. 30, 2015	Jun. 30, 2015
Debt Instrument [Line Items]
Total	$ 611,000,000	$ 607,100,000
Less: amounts due within one year	0	0
Long-term debt, net of current portion	611,000,000	607,100,000
Medium-term notes, Series B 6.97% to 7.10% due April 2018 To May 2018 Member
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 Member | Minimum
Debt Instrument [Line Items]
Interest rate (as a percent)	6.97%	6.97%
Medium-term notes, Series B 6.97% to 7.10% due April 2018 To May 2018 Member | Maximum
Debt Instrument [Line Items]
Interest rate (as a percent)	7.10%	7.10%
Senior unsecured notes, 5.20% due July 2021
Debt Instrument [Line Items]
Face amount	$ 250,000,000.0	$ 250,000,000.0
Interest rate (as a percent)	5.20%	5.20%
Total	$ 256,300,000	$ 252,500,000
4.45% Senior Unsecured Notes due 2023
Debt Instrument [Line Items]
Face amount	$ 300,000,000.0	$ 300,000,000.0
Interest rate (as a percent)	4.45%	4.45%
Total	$ 299,700,000	$ 299,600,000