Investment in Piceance Energy Summarized Financial Information (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
ASSETS
Current assets	$ 436,883		$ 436,883		$ 460,789
Current liabilities	390,299		390,299		310,806
Natural gas and oil revenues	1,709	$ 1,839	2,185	$ 3,416
Income (loss) from operations	27,460	(24,380)	45,317	(39,182)
Net income (loss)	11,723	(24,677)	12,185	(39,245)
Piceance Energy [Member]
ASSETS
Current assets	6,396		6,396		13,168
Non-current assets	481,955		481,955		468,379
Current liabilities	16,347		16,347		17,103
Non-current liabilities	55,118		55,118		$ 107,087
Natural gas and oil revenues	10,486	20,725	21,223	40,968
Income (loss) from operations	(8,207)	1,989	(15,286)	5,598
Net income (loss)	$ (9,329)	$ 1,815	$ (15,409)	$ 724