Equity (Details) - USD ($)		12 Months Ended
	Sep. 09, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Aug. 31, 2012
Class of Stock [Line Items]
Common stock, shares authorized		280,000,000	280,000,000
Preferred stock, shares authorized		50,000,000	50,000,000
Common stock shares issued in connection with stock options exercised		281,091
Common stock shares issued through dividend reinvestment plan		2,142	2,434	2,002
Number of shares of stock grants withheld		45,090	55,523	48,310
Number of shares forfeited		1,529	7,970	7,653
Common shares issued under Employee Stock Purchase Plan		10,667
Proceeds from issuance of common stock		$ 690,184,000	$ 346,134,000	$ 4,703,000
Forward Contracts [Member]
Class of Stock [Line Items]
Proceeds from issuance of common stock		$ 659,423,000
Market closing price (in dollars per share)	$ 155.83
Option Indexed to Issuer's Equity, Shares	4,500,000	4,500,000
Forward Contract Indexed To Issuers Equity, Weighted Average Settlement Price Per Share		$ 146.54
Forward rate (in dollars per share)	$ 151.74
Continuous Equity Program CEP III [Member]
Class of Stock [Line Items]
Common stock shares issued		0
Maximum value of shares of common stock that can be sold					$ 750,000,000
Period during which common stock can be sold (in months)		36 months
Continuous Equity Program CEP IV [Member]
Class of Stock [Line Items]
Common stock shares issued		0
Maximum value of shares of common stock that can be sold		$ 1,000,000,000
Percentage of compensation received by sales agent		2.00%
Maximum [Member] | Continuous Equity Program CEP IV [Member]
Class of Stock [Line Items]
Percentage of compensation received by sales agent		2.00%
Restricted Stock Converted From Performance Shares [Member]
Class of Stock [Line Items]
Common stock shares issued in connection with stock grants		157,779
Deferred Compensation, Share-based Payments [Member] | Non Employee Director [Member]
Class of Stock [Line Items]
Conversion of restricted stock units (in shares)		46,589