Stockholder's Equity (Details 1) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	652,724	429,058
Dividend reinvestment plan issuances, Average Price	$ 61.59	$ 54.61
Dividend reinvestment plan issuances, Net Proceeds	40,199,000	23,429,000
Dividend reinvestment plan issuances, Gross Proceeds	40,199,000	23,429,000
Option exercises, Shares	119,999	43,047
Option exercises, Average Price	43.15	37.42
Option exercises, Gross Proceeds	5,178,000	1,611,000
Option exercises, Net Proceeds	5,178,000	1,611,000
Issuance of Common Stock, Shares	772,723	21,172,105
Senior note conversions	0
Gross Proceeds From Issuance of Common Stock	45,377,000	1,132,490,000
Net proceeds from the issuance of common stock	45,377,000	1,087,777,000
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		$ 1,062,737,000