Equity (Details 2) (USD $)	3 Months Ended			1 Months Ended	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Aug. 31, 2012 CEP III	Mar. 31, 2014 CEP III
Equity
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	84,836
Common stock shares issued through dividend reinvestment plan	638	550
Common stock shares issued in connection with stock grants	104,060	119,292
Number of shares of stock grants withheld	33,365	29,219
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
Value of additional shares authorized for issuance					$ 646,274,000