Long-Term Debt (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Senior notes	$ 1,384,556,000	$ 1,384,488,000
Current portion of debt	(884,881,000)	0
Debt (less current portion)	499,675,000	1,384,488,000
Fair value of long-term debt	1,425,056,000	1,429,921,000
Maximum borrowing capacity on Competitive Advance and Revolving Credit Facility	650,000,000	550,000,000
Expiry date of revolving credit facility	Mar. 31, 2019	Jun. 30, 2014
Interest rate, variable rate basis on drawn amounts of revolving credit facility	LIBOR
Ownership interest (in hundredths)	35.00%
Basis spread on variable rate (in hundredths)	0.90%
Basis spread on variable rate, commitment fee (in hundredths)	0.10%
Outstanding borrowings at period end	0	0
Senior notes due in 2015 [Member]
Debt Instrument [Line Items]
Senior notes	884,881,000	884,844,000
Stated interest rate (in hundredths)	3.55%
Maturity date	Jan. 15, 2015
Face value	885,000,000
Senior notes due in 2016 [Member]
Debt Instrument [Line Items]
Senior notes	499,675,000	499,644,000
Stated interest rate (in hundredths)	2.70%
Maturity date	Dec. 15, 2016
Face value	$ 500,000,000