Stockholder's Equity (Details 1) (USD $)	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued		743,099
Equity shelf plan issuances, average price		$ 50.59
Equity Shelf Plan Issuances Gross Proceeds		37,595,000
Equity Shelf Plan Issuances Net Proceeds		36,870,000
Dividend reinvestment plan issuances, Shares Issued	1,485,598	1,869,796
Dividend reinvestment plan issuances, Average Price	55.65	48.39
Dividend reinvestment plan issuances, Net Proceeds	82,678,000	89,528,000
Dividend reinvestment plan issuances, Gross Proceeds	82,678,000	90,476,000
Option exercises, Shares	155,195	151,927
Option exercises, Average Price	39.61	37.78
Option exercises, Gross Proceeds	6,147,000	5,740,000
Option exercises, Net Proceeds	6,147,000	5,740,000
Issuance of Common Stock, Shares	67,080,514	31,514,822
Senior note conversions	1,039,721
Gross Proceeds From Issuance of Common Stock	3,681,425,000	1,549,749,000
Net proceeds from the issuance of common stock	3,536,232,000	1,490,681,000
March 2011 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued		28,750,000
Public issuance, Average Price		49.25
Public issuance, Gross Proceeds		1,415,938,000
Public issuance, Net Proceeds		1,358,543,000
February 2012 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued	20,700,000
Public issuance, Average Price	53.50
Public issuance, Gross Proceeds	1,107,450,000
Public issuance, Net Proceeds	1,062,256,000
August 2012 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued	13,800,000
Public issuance, Average Price	58.75
Public issuance, Gross Proceeds	810,750,000
Public issuance, Net Proceeds	778,011,000
September 2012 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued	29,900,000
Public issuance, Average Price	56.00
Public issuance, Gross Proceeds	1,674,400,000
Public issuance, Net Proceeds	$ 1,607,140,000