Equity and Capital (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 01, 2013
Targeted or Tracking Stock, Stock [Line Items]
Common stock offering, maximum aggregate price			$ 300,000,000
Commission to sales agent, maximum (percent)			2.00%
Number of shares settled	269,929	238,693
Common stock offering, maximum aggregate price still available	$ 88,507,000
At The Market Stock Sales
Targeted or Tracking Stock, Stock [Line Items]
Number of shares settled	8,419,298.000
Weighted-average sales price (in usd per share)	$ 25.12