Debt (Details) (USD $)	0 Months Ended				12 Months Ended					1 Months Ended
	Aug. 15, 2011	Feb. 15, 2011	Aug. 15, 2010	Feb. 15, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Feb. 29, 2004	Dec. 31, 2011 Letters Of Credit [Member]	Feb. 29, 2004 First Series [Member]	Feb. 29, 2004 Second Series [Member]	Dec. 31, 2011 Credit Facility 2009 [Member]	Dec. 31, 2011 Credit Facility 2011 [Member]	Dec. 18, 2009 Credit Facility 2011 [Member]	Dec. 31, 2011 Credit Facility 2011 [Member] Letters Of Credit [Member]
Line of Credit Facility [Line Items]
Senior notes with an aggregate principal amount								$ 975,000,000
Net principal amount								971,537,000		500,000,000	475,000,000
Senior notes amortized discount								3,463,000
Senior notes interest rate										5.63%	6.75%
Senior notes discount rate										0.11%	0.61%
Senior notes maturity date										Feb. 15, 2014	Feb. 15, 2034
Senior notes interest expenses					60,188,000	60,188,000	60,188,000
Senior notes accrued interest					22,570,000	22,570,000	22,570,000
Senior notes interest payment	30,094,000	30,094,000	30,094,000	30,094,000
Senior revolving credit facility												350,000,000	350,000,000		24,296,000
Sublimit for letters of credit issued														50,000,000
Commercial paper outstanding					0	0
Maximum borrowing capacity													525,000,000
Borrowing credit facility												0
Credit facility outstanding									$ 325,704,000