Equity (Details 2) - USD ($)		1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 09, 2014	Aug. 31, 2012	Sep. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Class of Stock [Line Items]
Common stock, shares authorized			280,000,000	280,000,000		280,000,000
Preferred stock, shares authorized			50,000,000	50,000,000		50,000,000
Common stock shares issued in connection with stock options exercised				155,173
Common stock issued through the dividend reinvestment plan (in shares)				1,608	1,868
Stock issued in connection with stock grants (in shares)				157,779	113,822
Number of shares of stock grants withheld				39,800	53,983
Stock Issued During Period, Shares, Employee Stock Purchase Plans				5,022
Maximum value of shares of common stock that can be sold (in dollars)		$ 750,000,000
Period during which common stock can be sold		36 months
Percentage of compensation received by sales agent				1.50%
Common stock shares issued			0	0
Issuance of common stock				$ 674,631,000	$ 340,091,000
Non Employee Director [Member] | Deferred Compensation, Share-based Payments [Member]
Class of Stock [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Conversion of Shares				46,589
Forward Contracts [Member]
Class of Stock [Line Items]
Forward Contract Indexed to Issuer's Equity, Shares	4,500,000			4,500,000
Share Price	$ 155.83
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share	$ 151.74
Forward Contract Indexed to Issuers Equity, Shares Issued in Settlement			3,890,725
Forward Contract Indexed To Issuers Equity, Settlement Price Per Share			$ 146.35
Issuance of common stock			$ 569,423,000	$ 659,423,000