Equity (Details 2) (USD $)	6 Months Ended			1 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Aug. 31, 2012 CEP III	Jun. 30, 2014 CEP III
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	224,771
Common stock shares issued through dividend reinvestment plan	1,286	1,030
Common stock shares issued in connection with stock grants	113,822	123,977
Common shares through the Employee Stock Purchase Program	4,367
Number of shares of stock grants withheld	50,105	44,222
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					1,418,959
Average sales price (in usd per share)					$ 140.94
Issuance of common stock	214,970,000	2,605,000			196,984,000
Value of additional shares authorized for issuance					$ 446,286,000