Investment in Piceance Energy Change in Equity Investment (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning balance			$ 104,657
Equity loss from Piceance Energy	$ (2,950)	$ 760	(4,776)	$ 539
Ending balance	127,410		127,410
Piceance Energy [Member]
Investments in and Advances to Affiliates, at Fair Value [Roll Forward]
Beginning balance			104,657
Equity loss from Piceance Energy			(5,186)
Accretion of basis difference			410
Capital contributions			27,529
Ending balance	$ 127,410		$ 127,410