Stockholder's Equity (Details 2) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of common stock issuances
Dividend reinvestment plan issuances, Shares Issued	4,145,457	4,024,169	4,122,941
Dividend reinvestment plan issuances, Average Price	$ 70.34	$ 67.72	$ 62.35
Dividend reinvestment plan issuances, Gross Proceeds	$ 291,852	$ 272,531	$ 257,055
Dividend reinvestment plan issuances, Net Proceeds	$ 291,571	$ 272,531	$ 257,055
Equity shelf program issuances, Shares Issued	3,134,901	696,070
Equity shelf program issuances, Average Price	$ 75.27	$ 69.23
Equity shelf program issuances, Gross Proceeds	$ 238,286	$ 48,186
Equity shelf program issuances, Net Proceeds	$ 235,959	$ 47,463
Option exercises, Shares	141,405	249,054	498,549
Option exercises, Average Price	$ 47.13	$ 47.35	$ 45.79
Option exercises, Gross Proceeds	$ 6,664	$ 11,793	$ 22,831
Option exercises, Net Proceeds	$ 6,664	$ 11,793	$ 22,831
Senior note conversions		1,330,474	258,542
Preferred stock conversions			233,236
Issuance of Common Stock, Shares	7,824,503	25,987,604	39,226,415
Gross Proceeds From Issuance Of Common Stock	$ 536,802	$ 1,808,535	$ 2,420,065
Net proceeds from the issuance of common stock	$ 534,194	$ 1,755,722	$ 2,343,868
Stock incentive plans, net of forfeitures, Shares Issued	402,740	137,837	188,147
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