Debt (Narrative) (Details) (USD $)	1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Feb. 28, 2011	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 4.10% Senior Notes Due 2022 [Member]	Mar. 31, 2012 4.10% Senior Notes Due 2022 [Member]	Aug. 02, 2012 Credit Facility [Member]	Jun. 30, 2012 Credit Facility [Member]	Feb. 28, 2011 Credit Facility [Member]
Total long-term debt				$ 1,668,811,000	$ 1,342,100,000	$ 1,000,000,000	$ 1,000,000,000
Debt instrument, interest rate, stated percentage						4.10%
Senior notes redemption date	Dec. 15, 2021		Feb. 01, 2016
Principal amount of notes to be redeemed then outstanding	100.00%
Duration of assumed year, days	360 days
Duration of assumed month, days		30 days
Basis Points				35				200	200
Purchase price upon exercise of option	101.00%
Unsecured revolving credit facility, borrowing capacity			$ 1,500,000,000							$ 2,000,000,000
Unsecured revolving credit facility, variable rate			3.50%