Debt (Details) (USD $) | 3 Months Ended | 9 Months Ended | 12 Months Ended | 3 Months Ended | |
|---|---|---|---|---|---|
Jun. 30, 2011 | Sep. 30, 2011
New Revolving Credit Facility [Member] | Dec. 31, 2010
Existing Revolving Credit Facility [Member] | Jun. 30, 2011
Interest Rate Swaps on 6.15% Senior Notes Due 2019 [Member] | Jun. 30, 2011
Interest Rate Swaps on 5.9% Senior Notes Due 2018 [Member] | |
| Line of Credit Facility [Line Items] | |||||
| Maximum borrowing capacity of credit facility | $ 2,000,000,000 | $ 1,200,000,000 | |||
| Expiration of credit facility (in years) | 5Y | ||||
| Initiation date of credit facility | February 22, 2011 | July 2007 | |||
| Interest rate swaps [Abstract] | |||||
| Number of debt instruments related to interest rate swaps | 2 | ||||
| Derivative [Line Items] | |||||
| Notional amount | $ 600,000,000 | $ 400,000,000 | |||
| Fixed rate received (in hundredths) | 6.15% | 5.90% | |||
| Variable rate paid | floating rate of three-month LIBOR plus a spread | floating rate of three-month LIBOR plus a spread | |||