Stockholder's Equity (Details 1) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	1,370,661	993,634
Dividend reinvestment plan issuances, Average Price	$ 66.87	$ 54.34
Dividend reinvestment plan issuances, Net Proceeds	91,651,000	53,991,000
Dividend reinvestment plan issuances, Gross Proceeds	91,651,000	53,991,000
Option exercises, Shares	186,404	104,574
Option exercises, Average Price	42.64	38.42
Option exercises, Gross Proceeds	7,948,000	4,018,000
Option exercises, Net Proceeds	7,948,000	4,018,000
Issuance of Common Stock, Shares	24,557,083	22,203,460
Senior note conversions	18
Gross Proceeds From Issuance of Common Stock	1,790,099,000	1,165,459,000
Net proceeds from the issuance of common stock	1,730,235,000	1,120,265,000
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