Investment in Laramie Energy - Summarized Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Current assets	$ 403,108				$ 531,752				$ 403,108	$ 531,752
Current liabilities	382,765				365,040				382,765	365,040
Income (loss) from operations	28,325	$ (21,784)	$ (3,313)	$ (19,719)	(10,077)	$ 26,274	$ 27,460	$ 17,857	(16,494)	61,514	$ (37,532)
Net income (loss)	13,687	$ (27,761)	$ (13,088)	$ (18,673)	(66,836)	$ 14,740	$ 11,723	$ 462	(45,835)	(39,911)	(47,041)
Laramie Energy Company
ASSETS
Current assets	12,199				8,511				12,199	8,511
Non-current assets	655,022				514,206				655,022	514,206
Current liabilities	58,067				18,158				58,067	18,158
Non-current liabilities	$ 186,631				$ 98,624				186,631	98,624
Natural gas and oil revenues									104,826	42,870	80,471
Income (loss) from operations									(27,325)	(40,984)	3,512
Net income (loss)									$ (61,849)	$ (49,159)	$ 6,576