Equity (Details 2) - USD ($)		1 Months Ended	3 Months Ended	6 Months Ended
	Sep. 09, 2014	Aug. 31, 2012	Jun. 30, 2015	Jun. 30, 2015	Jun. 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				54,196
Common stock issued through the dividend reinvestment plan (in shares)				1,028	1,286
Stock issued in connection with stock grants (in shares)				157,779	113,822
Number of shares of stock grants withheld				36,104	50,105
Stock Issued During Period, Shares, Employee Stock Purchase Plans				5,022
Issuance of common stock				$ 97,326,000	$ 214,970,000
Share Price	$ 155.83
Forward Contract Indexed to Issuer's Equity, Shares	4,500,000			609,275
Forward Contract Indexed to Issuer's Equity, Forward Rate Per Share	$ 151.74		$ 147.72
CEP III
Class of Stock [Line Items]
Common stock shares issued			0	0
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%
Value of additional shares authorized for issuance			$ 346,304,000	$ 346,304,000
Forward Equity Contract [Member]
Class of Stock [Line Items]
Issuance of common stock			$ 90,000,000
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