Stockholder's Equity (Details 1) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued	743,099	431,082
Equity shelf plan issuances, average price	50.59	44.94
Equity shelf plan issuances, gross proceeds	$ 37,595	$ 19,371
Equity shelf plan issuances, net proceeds	36,870	19,014
Dividend reinvestment plan issuances, Shares Issued	1,869,796	1,441,612
Dividend reinvestment plan issuances, Average Price	48.39	42.83
Dividend reinvestment plan issuances, Net Proceeds	89,528	61,737
Dividend reinvestment plan issuances, Gross Proceeds	90,476	61,737
Option exercises, Shares	151,927	56,947
Option exercises, Average Price	37.78	33.24
Option exercises, Gross Proceeds	5,740	1,893
Option exercises, Net Proceeds	5,740	1,893
Issuance of Common Stock, Shares	31,514,822	11,129,641
Gross Proceeds From Issuance of Common Stock	1,549,749	503,901
Net proceeds from the issuance of common stock	1,490,681	486,565
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	1,358,543
September 2010 public issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued		9,200,000
Public issuance, Average Price		45.75
Public issuance, Gross Proceeds		420,900
Public issuance, Net Proceeds		$ 403,921