Debt (Details) (USD $) In Millions, unless otherwise specified				1 Months Ended				1 Months Ended	12 Months Ended		12 Months Ended					12 Months Ended			12 Months Ended
	Jun. 30, 2012 Senior unsecured notes, 5.200% due July 2021	Jun. 30, 2011 Senior unsecured notes, 5.200% due July 2021	Jun. 28, 2011 Senior unsecured notes, 5.200% due July 2021	Aug. 31, 2011 Medium-term notes, Series C at 7.625% due August 2011	Jun. 30, 2012 Medium-term notes, Series C at 7.625% due August 2011	Jun. 30, 2011 Medium-term notes, Series C at 7.625% due August 2011	Jun. 28, 2011 Medium-term notes, Series C at 7.625% due August 2011	Jun. 30, 2011 Credit Agreement	Jun. 30, 2012 Credit Agreement	Jun. 21, 2011 Credit Agreement	Jun. 30, 2012 Credit Agreement Minimum	Jun. 30, 2012 Credit Agreement Maximum	Jun. 30, 2012 Credit Agreement Base Rate	Jun. 30, 2012 Credit Agreement Base Rate Minimum	Jun. 30, 2012 Credit Agreement Base Rate Maximum	Jun. 30, 2012 Credit Agreement LIBOR	Jun. 30, 2012 Credit Agreement LIBOR Minimum	Jun. 30, 2012 Credit Agreement LIBOR Maximum	Jun. 30, 2012 Letters of credit	Jun. 30, 2012 Letters of credit Minimum	Jun. 30, 2012 Letters of credit Maximum
Debt
Face amount	$ 250.0	$ 250.0	$ 250.0			$ 100.0
Interest rate (as a percent)	5.20%	5.20%	5.20%		7.625%	7.625%	7.625%
Expected repayment of principal amount				100
Maximum borrowing capacity										350
Maturity term of debt instrument								5 years
Debt issuance costs								1.4
Variable rate basis													Base Rate			LIBOR
Interest rate margin (as a percent)													0.20%	0.00%	0.95%	1.20%	0.65%	1.95%
Credit facility, fees rate (as a percent)									0.20%		0.10%	0.45%
Letter of credit fees (as a percent)																			1.20%	0.65%	1.95%
Letters of credit issued																			5.9
Letters of credit available for future borrowings									$ 344.1
Required interest coverage ratio									3.5
Required debt to capital ratio									0.55