Stockholder's Equity (Details 1) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Summary of common stock issuances
Public issuance, Shares Issued	20,700,000	28,750,000
Public issuance, Average Price	$ 53.50	$ 49.25
Public issuance, Gross Proceeds	1,107,450,000	1,415,938,000
Public issuance, Net Proceeds	1,062,737,000	1,358,694,000
Dividend reinvestment plan issuances, Shares Issued	429,058	574,652
Dividend reinvestment plan issuances, Average Price	54.61	48.42
Dividend reinvestment plan issuances, Net Proceeds	(23,429,000)	(27,822,000)
Dividend reinvestment plan issuances, Gross Proceeds	(23,429,000)	(27,822,000)
Option exercises, Shares	43,047	37,922
Option exercises, Average Price	37.42	42.24
Option exercises, Gross Proceeds	(1,611,000)	(1,602,000)
Option exercises, Net Proceeds	1,611,000	1,602,000
Issuance of Common Stock, Shares	21,172,105	29,362,574
Gross Proceeds From Issuance of Common Stock	1,132,490,000	1,445,362,000
Net proceeds from the issuance of common stock	$ 1,087,777,000	$ 1,388,118,000