Summary of Restructuring Related Obligations (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Sep. 30, 2012	Dec. 31, 2011
Restructuring Reserve [Roll Forward]
Balance at beginning of period	$ 16,090		$ 16,972	$ 17,263
Employee, office and other costs accrued, net	12	0
Accretion of firm transportation obligation	354	207
Cash payments, net	(650)	(498)
Balance at end of period	$ 15,806		$ 16,972	$ 17,263