Other Expense, Income and Non-Operating Other Income, Net - Schedule of Other Income and Non-Operating Other Income, Net (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Other expense:
Realized and unrealized losses on contractual obligations associated with carbon credits	$ 2,799	$ 1,233	$ 721
Losses on sales and disposals of property and equipment	1,312	737	996
Dismantle and demolition expense	559	2,578
Miscellaneous penalties	18	824	112
Miscellaneous expense	197	302	289
Total other expense	4,885	5,674	2,118
Other income:
Realized and unrealized gains on carbon credits	3,089	1,233	876
Settlements of litigation and potential litigation		545	2,303
Miscellaneous income	676	545	632
Total other income	3,765	2,323	3,811
Other expense (income), net	1,120	3,351	(1,693)
Non-operating other expense (income), net:
Interest income	(301)	(165)	(87)
Miscellaneous income		(1)	(263)
Miscellaneous expense	20	66	69
Total non-operating other income, net	$ (281)	$ (100)	$ (281)