Summary of Restructuring Related Obligations (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Restructuring Reserve [Roll Forward]
Balance at beginning of period	$ 576	$ 64	$ 529
Employee, office and other costs accrued, net	1,284	2,351	8,200
Firm transportation charge	17,332	0	0
Accretion of obligations	570	0	0
Cash payments, net	(2,499)	(1,839)	(8,665)
Balance at end of period	$ 17,263	$ 576	$ 64