Debt (Details) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended	9 Months Ended
	Apr. 30, 2011	Dec. 31, 2011	Dec. 31, 2011	Dec. 31, 2011
Debt Instrument [Line Items]
Line of credit facility, repayments	$ 250,000,000
Amount available under line of credit		1,000,000,000	1,000,000,000	1,000,000,000
Line of credit facility maximum incremental credit increases		500,000,000	500,000,000	500,000,000
Interest rates applicable to Revolving Credit Facility
Line of Credit Facility, margin on base borrowing		0.25%	0.25%	0.25%
Applicable margin on Eurocurrency borrowing		1.10%	1.10%	1.10%
Facility commitment fee				0.15%
Payments of debt issuance costs			$ 2,000,000
Line of credit facility financial covenant ratio of consolidated debt to consolidated cash flow		400.00%
Line of credit facility financial covenant ratio of consolidated cash flow to sum of interest payable and amortization of debt discount		350.00%
Line of credit facility, financial covenants			The Revolving Credit Facility due August 2016 contains customary covenants for borrowings of this type, including two financial covenants: (i) for the 12 months ending each quarter-end, the ratio of consolidated debt for borrowed money to consolidated cash flow, each as defined in the facility Credit Agreement, must not exceed 4.00 to 1.00; and (ii) for the 12 months ending at any date, the ratio of consolidated cash flow to the sum of interest payable on, and amortization of debt discount in respect of, all consolidated debt for borrowed money, as defined in the facility Credit Agreement, must not be less than 3.50 to 1.00.
Line of credit facility, financial covenants compliance			At December 31, 2011, the Company was in compliance with all covenants.
Base Rate Borrowings [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate at period end		3.50%	3.50%	3.50%
Eurocurrency Rate Borrowings [Member]
Debt Instrument [Line Items]
Line of credit facility, interest rate at period end		1.40%	1.40%	1.40%