Common Stock Common Stock (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended	24 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 30, 2014	Mar. 31, 2014	Mar. 31, 2014	Jun. 30, 2012
Class of Stock [Line Items]
Combined common and preferred stock, shares authorized	250,000,000
Common stock, shares authorized	200,000,000	200,000,000
Common stock, par or stated value per share	$ 0.01	$ 0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, par or stated value per share	$ 0.01	$ 0.01
Common stock reserved for incentive plan awards	2,865,957
Common stock aggregate gross sales price, maximum	$ 400,000,000
Net proceeds from sale of stock	399,505,000	57,276,000	28,966,000
Shares paid for tax withholding	23,630	34,552
Hydro-electric Assets [Member]
Class of Stock [Line Items]
Stock issued during period, shares				7,766,990
Shares issued, price per share	$ 51.50
Net proceeds from sale of stock				386,000,000
Equity Distribution Agreement [Member]
Class of Stock [Line Items]
Common stock aggregate gross sales price, maximum							100,000,000
Net proceeds from sale of stock					13,400,000	98,700,000
Sales commission and other fees					$ 147,000
Number of shares issued during the period under the Equity Distribution Agreement					295,979	2,492,889
Common stock average share price					$ 45.65	$ 40.11