Debt (Details 2) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2014 Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018	Jun. 30, 2013 Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018	Jun. 30, 2014 Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018 Minimum	Jun. 30, 2013 Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018 Minimum	Jun. 30, 2014 Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018 Maximum	Jun. 30, 2013 Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018 Maximum	Jun. 30, 2014 Senior unsecured notes, 5.20% due July 2021	Jun. 30, 2013 Senior unsecured notes, 5.20% due July 2021	Jun. 30, 2014 4.45% Senior Unsecured Notes due 2023	Jun. 30, 2013 4.45% Senior Unsecured Notes due 2023	Mar. 31, 2013 4.45% Senior Unsecured Notes due 2023	Feb. 26, 2013 4.45% Senior Unsecured Notes due 2023
Long-term debt outstanding
Face amount				$ 55.0	$ 55.0					$ 250.0	$ 250.0	$ 300.0	$ 300.0	$ 300.0	$ 300.0
Interest rate (as a percent)						6.97%	6.97%	7.10%	7.10%	5.20%	5.20%	4.45%	4.45%		4.45%
Total	604.3	604.2		55.0	55.0					249.7	249.7	299.6	299.5
Long-term debt, net of current portion	604.3	604.2
Aggregate maturities of long-term debt, 2015	0
Aggregate maturities of long-term debt, 2016	0
Aggregate maturities of long-term debt, 2017	0
Aggregate maturities of long-term debt, 2018	55.0
Aggregate maturities of long-term debt, 2019	0
Aggregate maturities of long-term debt, thereafter	550.0
Interest costs	32.1	27.8	25.0
Interest costs, capitalized	$ 15.1	$ 6.8	$ 1.2