Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Mar. 31, 2014	Mar. 31, 2013	Jun. 30, 2013
Secured and unsecured debt obligations
Total	$ 604.3		$ 604.3		$ 604.2
Long-term debt, net of current portion	604.3		604.3		604.2
Interest costs	8.1	7.0	24.1	19.0
Interest costs, capitalized	5.4	2.0	13.3	4.3
Credit Agreement
Secured and unsecured debt obligations
Maximum borrowing capacity	500		500
Letters of credit issued	8.2		8.2
Credit Agreement available for future borrowings	491.8		491.8
Credit Agreement | LIBOR
Secured and unsecured debt obligations
Variable rate basis	LIBOR
Interest rate margin (as a percent)	1.25%
Credit Agreement | LIBOR | Minimum
Secured and unsecured debt obligations
Interest rate margin (as a percent)	0.75%
Credit Agreement | LIBOR | Maximum
Secured and unsecured debt obligations
Interest rate margin (as a percent)	1.90%
Credit Agreement | Base Rate
Secured and unsecured debt obligations
Variable rate basis	Base Rate
Interest rate margin (as a percent)	0.25%
Credit Agreement | Base Rate | Minimum
Secured and unsecured debt obligations
Interest rate margin (as a percent)	0.00%
Credit Agreement | Base Rate | Maximum
Secured and unsecured debt obligations
Interest rate margin (as a percent)	0.90%
Letters of credit
Secured and unsecured debt obligations
Commitment fee rate (as a percent)	0.15%
Letter of credit fees (as a percent)	1.25%
Required interest coverage ratio	3.50
Letters of credit | Minimum
Secured and unsecured debt obligations
Commitment fee rate (as a percent)	0.075%
Letter of credit fees (as a percent)	0.75%
Letters of credit | Maximum
Secured and unsecured debt obligations
Commitment fee rate (as a percent)	0.375%
Letter of credit fees (as a percent)	1.90%
Required debt to capital ratio	0.55		0.55
Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018
Secured and unsecured debt obligations
Face amount	55.0		55.0		55.0
Total	55.0		55.0		55.0
Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018 | Minimum
Secured and unsecured debt obligations
Interest rate (as a percent)	6.74%		6.74%		6.74%
Medium-term notes, Series B at 6.74% to 7.10% due from April 2018 to May 2018 | Maximum
Secured and unsecured debt obligations
Interest rate (as a percent)	7.10%		7.10%		7.10%
Senior unsecured notes, 5.20% due July 2021
Secured and unsecured debt obligations
Interest rate (as a percent)	5.20%		5.20%		5.20%
Face amount	250.0		250.0		250.0
Total	249.7		249.7		249.7
Senior unsecured notes, 4.45% due March 2023
Secured and unsecured debt obligations
Interest rate (as a percent)	4.45%		4.45%		4.45%
Face amount	300.0	300.0	300.0	300.0	300.0
Total	$ 299.6		$ 299.6		$ 299.5