Investment in Laramie Energy Summarized Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
ASSETS
Current assets	$ 461,386		$ 403,108
Current liabilities	422,509		382,765
REVENUE
Income (loss) from operations	29,189	$ (19,719)
Net income (loss)	27,786	(18,673)
Laramie Energy Company
ASSETS
Current assets	10,084		12,199
Non-current assets	654,719		655,022
Current liabilities	41,336		58,067
Non-current liabilities	181,746		$ 186,631
REVENUE
Natural gas and oil revenues	40,612	14,693
Income (loss) from operations	1,163	(11,124)
Net income (loss)	$ 17,528	$ (2,579)