Stockholder's Equity (Details 1) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	1,200,418	855,343
Dividend reinvestment plan issuances, Average Price	49.44	41.59
Dividend reinvestment plan issuances, Gross Proceeds	$ 59,348	$ 35,570
Dividend reinvestment plan issuances, Net Proceeds	58,400	35,570
Option exercises, Shares	116,782	51,313
Option exercises, Average Price	37.69	38.78
Option exercises, Gross Proceeds	4,401	1,990
Option exercises, Net Proceeds	4,401	1,990
Issuance of Common Stock, Shares	30,067,200	906,656
Gross Proceeds From Issuance of Common Stock	1,479,687	37,560
Net proceeds from the issuance of common stock	1,421,495	37,560
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
Public issuance, Net Proceeds	$ 1,358,694