Stockholder's Equity (Details 1)	9 Months Ended
	Sep. 30, 2015 USD ($) $ / shares shares	Sep. 30, 2014 USD ($) $ / shares shares
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued | shares	2,935,950	3,114,052
Dividend reinvestment plan issuances, Average Price	70.28	60.05
Dividend reinvestment plan issuances, Net Proceeds	$ 206,334,000	$ 186,996,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 206,334,000	$ 186,996,000
Option exercises, Shares | shares	247,005	207,046
Option exercises, Average Price	47.42	45.94
Option exercises, Gross Proceeds	$ 11,712,000	$ 9,512,000
Option exercises, Net Proceeds	$ 11,712,000	$ 9,512,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	144,779	186,837
Issuance of Common Stock, Shares | shares	24,208,208	37,866,114
Senior note conversions | shares	1,330,474	199,943
Equity issued in acquisition of noncontrolling interest | shares		233,236
Gross Proceeds From Issuance of Common Stock	$ 1,694,071,000	$ 2,336,687,000
Net proceeds from the issuance of common stock	$ 1,641,981,000	$ 2,260,908,000
June 2014 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued | shares		16,100,000
Public issuance, Average Price | $ / shares		62.35
Public issuance, Gross Proceeds		$ 1,003,835,000
Public issuance, Net Proceeds		$ 968,517,000
September 2014 Public Issuance [Member]
Summary of common stock issuances
Public issuance, Shares Issued | shares		17,825,000
Public issuance, Average Price | $ / shares		63.75
Public issuance, Gross Proceeds		$ 1,136,344,000
Public issuance, Net Proceeds		$ 1,095,883,000
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