Stockholder's Equity (Details 1)	6 Months Ended
	Jun. 30, 2015 USD ($) $ / shares shares	Jun. 30, 2014 USD ($) shares
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued | shares	1,766,585	2,132,427
Dividend reinvestment plan issuances, Average Price	72.83	58.73
Dividend reinvestment plan issuances, Net Proceeds	$ 128,653,000	$ 125,232,000
Dividend reinvestment plan issuances, Gross Proceeds	$ 128,653,000	$ 125,232,000
Option exercises, Shares | shares	211,041	175,699
Option exercises, Average Price	46.26	46.78
Option exercises, Gross Proceeds	$ 9,762,000	$ 8,220,000
Option exercises, Net Proceeds	$ 9,762,000	$ 8,220,000
Stock incentive plans, net of forfeitures, Shares Issued | shares	179,037
Issuance of Common Stock, Shares | shares	22,783,634	18,766,086
Senior note conversions | shares	1,076,971	1
Equity issued in acquisition of noncontrolling interest | shares		233,236
Gross Proceeds From Issuance of Common Stock	$ 1,614,440,000	$ 1,137,287,000
Net proceeds from the issuance of common stock	$ 1,562,350,000	$ 1,101,969,000
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