Stockholder's Equity (Details 2) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	4,024,169	4,122,941	3,429,928
Dividend reinvestment plan issuances, Average Price	$ 67.72	$ 62.35	$ 62.78
Dividend reinvestment plan issuances, Gross Proceeds	$ 272,531	$ 257,055	$ 215,346
Dividend reinvestment plan issuances, Net Proceeds	$ 272,531	$ 257,055	$ 215,346
Equity shelf program issuances, Shares Issued	696,070
Equity shelf program issuances, Average Price	$ 69.23
Equity shelf program issuances, Gross Proceeds	$ 48,186
Equity shelf program issuances, Net Proceeds	$ 47,463
Option exercises, Shares	249,054	498,549	213,724
Option exercises, Average Price	$ 47.35	$ 45.79	$ 42.16
Option exercises, Gross Proceeds	$ 11,793	$ 22,831	$ 9,010
Option exercises, Net Proceeds	$ 11,793	$ 22,831	$ 9,010
Senior note conversions	1,330,474	258,542	988,007
Equity issued in acquisition of noncontrolling interest			1,108,917
Preferred stock conversions		233,236	116,618
Issuance of Common Stock, Shares	25,987,604	39,226,415	28,857,194
Gross Proceeds From Issuance Of Common Stock	$ 1,808,535	$ 2,420,065	$ 1,914,856
Net proceeds from the issuance of common stock	$ 1,755,722	$ 2,343,868	$ 1,854,637
Stock incentive plans, net of forfeitures, Shares Issued	137,837	188,147
May 2013 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues			23,000,000
Public issuance, Average Price			$ 73.5
Public issuance, Gross Proceeds			$ 1,690,500
Public issuance, Net Proceeds			$ 1,630,281
June 2014 public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues		16,100,000
Public issuance, Average Price		$ 62.35
Public issuance, Gross Proceeds		$ 1,003,835
Public issuance, Net Proceeds		$ 968,517
September 2014 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues		17,825,000
Public issuance, Average Price		$ 63.75
Public issuance, Gross Proceeds		$ 1,136,344
Public issuance, Net Proceeds		$ 1,095,465
February 2015 Public Issuance [Member]
Summary of common stock issuances
Stock Issued During Period, Shares, New Issues	19,550,000
Public issuance, Average Price	$ 75.5
Public issuance, Gross Proceeds	$ 1,476,025
Public issuance, Net Proceeds	$ 1,423,935