Debt (Details 2) - USD ($) $ in Millions	12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2013	Feb. 26, 2013
Long-term debt outstanding
Total	$ 607.1	$ 604.3
Less amounts due within one year	0.0	0.0
Long-term debt, net of current portion	607.1	604.3
Aggregate maturities of long-term debt, 2016	0.0
Aggregate maturities of long-term debt, 2017	0.0
Aggregate maturities of long-term debt, 2018	55.0
Aggregate maturities of long-term debt, 2019	0.0
Aggregate maturities of long-term debt, 2020	0.0
Aggregate maturities of long-term debt, thereafter	550.0
Interest costs	30.4	32.1	$ 27.8
Interest costs, capitalized	2.7	15.1	$ 6.8
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, 2015 and 2014)
Long-term debt outstanding
Face amount	55.0	55.0
Total	$ 55.0	$ 55.0
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, 2015 and 2014) | Minimum
Long-term debt outstanding
Interest rate	6.97%	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, 2015 and 2014) | Maximum
Long-term debt outstanding
Interest rate	7.10%	7.10%
Senior unsecured notes, 5.20% due July 2021 (face value of $250.0 million at June 30, 2015 and 2014)
Long-term debt outstanding
Face amount	$ 250.0	$ 250.0
Interest rate	5.20%	5.20%
Total	$ 252.5	$ 249.7
Senior unsecured notes, 4.45% due March 2023 (face value of $300.0 million at June 30, 2015 and 2014)
Long-term debt outstanding
Face amount	$ 300.0	$ 300.0		$ 300.0
Interest rate	4.45%	4.45%		4.45%
Total	$ 299.6	$ 299.6
Credit Agreement
Long-term debt outstanding
Maturity term of debt instrument	5 years