Subsequent Events (Details) (USD $)	6 Months Ended	1 Months Ended		1 Months Ended		6 Months Ended
	Jun. 30, 2011	Jul. 31, 2011 At-the-market equity offering programs Issuance of common stock	Jul. 27, 2011 At-the-market equity offering programs Issuance of common stock	Aug. 31, 2011 Offering of debt 5.00% senior notes due on August 15, 2018	Aug. 02, 2011 Offering of debt 5.00% senior notes due on August 15, 2018	Jun. 30, 2011 At-the-market equity offering programs
Subsequent Events
Shares of common stock sold (in shares)		1,200,000				5,600,000
Aggregate proceeds from shares sold		$ 98,600,000				$ 426,400,000
Net proceeds from shares sold after related expenses	419,463,000	97,100,000				420,000,000
Aggregate value of the shares of common stock to be sold			250,000,000			525,000,000
Aggregate principal amount					250,000,000
Proceeds from Debt, Net of Issuance Costs				$ 246,500,000
Interest rate (as a percent)					5.00%