Quarterly Financial Data (unaudited) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 15,676	$ 23,947	$ 68,164	$ 594,086	$ 33,868	$ 36,827	$ 33,788	$ 26,773	$ 701,873	$ 131,256	$ 139,396
Operating income (loss)	(15,946)	(1,335)	21,038	509,493	(1,964)	3,383	1,583	(2,225)	513,250	777	2,134
Net income (loss) attributable to the Company	$ (11,101)	$ (50)	$ 14,609	$ 402,995	$ 562	$ 4,198	$ 2,704	$ (2,474)	$ 406,453	$ 4,990	$ 6,012
Basic and diluted income (loss) per share attributable to the Company (in dollars per share)	$ (0.13)	$ 0.00	$ 0.16	$ 4.37	$ 0.00	$ 0.05	$ 0.03	$ (0.03)	$ 4.40	$ 0.05	$ 0.07