Segment Disclosures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2014 item	Mar. 31, 2013 item	Mar. 31, 2012 item
Segment Disclosures
Number of reportable segment									1	1	1
Segment disclosures
Total revenues	$ 87,497	$ 25,847	$ 36,863	$ 57,189	$ 45,106	$ 73,831	$ 24,545	$ (2,787)	$ 207,396	$ 140,695	$ 268,581
Long-lived assets	1,237,248				1,270,024				1,237,248	1,270,024
U.S.
Segment disclosures
Net realized revenues									57,628	70,161	70,928
Net unrealized revenues									142	(37,139)	30,489
Long-lived assets	397,331				411,420				397,331	411,420
Canada
Segment disclosures
Net realized revenues									158,500	160,408	114,460
Net unrealized revenues									(8,874)	(52,735)	52,704
Long-lived assets	$ 839,917				$ 858,604				$ 839,917	$ 858,604