Capital Stock - Issuances of Common Stock (Details) - USD ($)	3 Months Ended						12 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock Issued During Period Shares New Issues									2,360,000
Stock Issued During Period, Value, New Issues							$ (83,275,000)	$ (171,295,000)	$ (35,400,000)
January 2014 Follow On Offering [Member]
Share Price						$ 12.5
Stock Issued During Period Shares New Issues						2,070,000
Stock Issued During Period, Value, New Issues						$ 24,174,000
March 2014 Follow On Offering [Member]
Share Price						$ 12.55
Stock Issued During Period Shares New Issues						3,680,000
Stock Issued During Period, Value, New Issues						$ 43,989,000
Underwriters Overallotment Proceeds Next Period						$ 5,700,000
Underwriters Overallotement Shares Next Period						480,000
At the Market Offering Program [Member]
Share Price	$ 13.65	$ 13.66	$ 13.87	$ 13.99	$ 13.14			$ 13.87
Stock Issued During Period Shares New Issues	5,024,530	1,210,487	3,675,207	3,389,441	537,499
Stock Issued During Period, Value, New Issues	$ 67,100,000	$ 16,175,000	$ 49,846,000	$ 46,372,000	$ 6,914,000
Value of Common Stock Available to be issued under At The Money Program							$ 15,000,000