Debt (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended	9 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012	Mar. 31, 2011
Line Of Credit Facility [Line Items]
Outstanding borrowings	$ 0		$ 0	$ 0
Line of Credit Facility, Initiation Date	May 10, 2011
Aggregate commitments including borrowings and letters of credit	400		400
Line of credit facility, annual fee, percentage			1.75%
Commitment fee percentage		0.20%
Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Borrowings and letters of credit, maturity date			May 2016
Maximum borrowings and letters of credit			550.0
Letter of credit, outstanding	9.3		9.3
Previous Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Aggregate commitments including borrowings and letters of credit	175		175
Borrowings and letters of credit, maturity date			January 2012
Letter of credit, outstanding				$ 3.0
Prime Rate [Member]
Line Of Credit Facility [Line Items]
Percentage of maximum raise in interest rate over base rate			1.50%
Prime Rate [Member] | Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Addition to LIBOR to determine interest rate		0.75%
LIBOR [Member]
Line Of Credit Facility [Line Items]
Percentage of maximum raise in interest rate over base rate			2.50%
LIBOR [Member] | Revolving Credit Facility [Member]
Line Of Credit Facility [Line Items]
Addition to LIBOR to determine interest rate		1.75%