Long-Term Debt (Details) (USD $)	9 Months Ended		9 Months Ended			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Senior notes due in 2015 [Member]	Dec. 31, 2011 Senior notes due in 2015 [Member]	Dec. 15, 2009 Senior notes due in 2015 [Member]	Sep. 30, 2012 Senior notes due in 2016 [Member]	Dec. 31, 2011 Senior notes due in 2016 [Member]	Dec. 01, 2011 Senior notes due in 2016 [Member]
Debt Instrument [Line Items]
Senior notes	$ 1,384,148,000	$ 1,383,945,000	$ 884,657,000	$ 884,545,000		$ 499,491,000	$ 499,400,000
Stated interest rate (in hundredths)					3.55%			2.70%
Maturity date			Jan. 15, 2015			Dec. 15, 2016
Fair value of long-term debt	1,456,506,000	1,428,653,000
Face value					885,000,000			500,000,000
Maximum borrowing capacity on Competitive Advance and Revolving Credit Facility	$ 550,000,000
Expiry date of revolving credit facility	June 2014