Stockholder's Equity (Details 1)	9 Months Ended
	Sep. 30, 2016 USD ($) $ / shares shares	Sep. 30, 2015 USD ($) $ / shares shares
Summary of common stock issuances
Equity shelf plan issuances, Shares Issued | shares	3,119,801
Equity shelf plan issuances, average price | $ / shares	75.27
Equity Shelf Plan Issuances Gross Proceeds	$ 237,131,000
Equity Shelf Plan Issuances Net Proceeds	$ 234,812,000
Dividend reinvestment plan issuances, Shares Issued | shares	3,946,821	2,935,950
Dividend reinvestment plan issuances, Average Price	70.51	70.28
Dividend reinvestment plan issuances, Net Proceeds	$ 278,297,000	$ 206,334,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 278,578,000	$ 206,334,000
Option exercises, Shares | shares	137,579	247,005
Option exercises, Average Price	50.57	47.42
Option exercises, Gross Proceeds	$ 6,958,000	$ 11,712,000
Option exercises, Net Proceeds	$ 6,958,000	$ 11,712,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	442,899	144,779
Senior note conversions | shares		1,330,474
Issuance of Common Stock, Shares | shares	7,647,100	24,208,208
Gross Proceeds From Issuance of Common Stock	$ 522,667,000	$ 1,694,071,000
Net proceeds from the issuance of common stock	$ 520,067,000	$ 1,641,981,000
February 2015 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued | shares		19,550,000
Public issuance, Average Price | $ / shares		75.5
Public issuance, Gross Proceeds		$ 1,476,025,000
Public issuance, Net Proceeds		$ 1,423,935,000