Derivatives, Hedges, Financial Instruments and Carbon Credits - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Recurring [Member], USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Assets-Supplies, prepaid items and other:
Total	$ 1,797	$ 1,315
Liabilities-Current and noncurrent accrued and other liabilities:
Total	2,649	2,524
Commodities contracts [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets	124	31
Carbon credits [Member]
Assets-Supplies, prepaid items and other:
Carbon credits	1,673	1,284
Liabilities-Current and noncurrent accrued and other liabilities:
Contractual obligations-carbon credits	1,673	1,284
Interest rate contracts [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities	976	1,240
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets-Supplies, prepaid items and other:
Total
Liabilities-Current and noncurrent accrued and other liabilities:
Total
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commodities contracts [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Carbon credits [Member]
Assets-Supplies, prepaid items and other:
Carbon credits
Liabilities-Current and noncurrent accrued and other liabilities:
Contractual obligations-carbon credits
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Interest rate contracts [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities
Significant Other Observable Inputs (Level 2) [Member]
Assets-Supplies, prepaid items and other:
Total	124
Liabilities-Current and noncurrent accrued and other liabilities:
Total	976
Significant Other Observable Inputs (Level 2) [Member] | Commodities contracts [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets	124
Significant Other Observable Inputs (Level 2) [Member] | Carbon credits [Member]
Assets-Supplies, prepaid items and other:
Carbon credits
Liabilities-Current and noncurrent accrued and other liabilities:
Contractual obligations-carbon credits
Significant Other Observable Inputs (Level 2) [Member] | Interest rate contracts [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities	976
Significant Unobservable Inputs (Level 3) [Member]
Assets-Supplies, prepaid items and other:
Total	1,673
Liabilities-Current and noncurrent accrued and other liabilities:
Total	1,673
Significant Unobservable Inputs (Level 3) [Member] | Commodities contracts [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets
Significant Unobservable Inputs (Level 3) [Member] | Carbon credits [Member]
Assets-Supplies, prepaid items and other:
Carbon credits	1,673
Liabilities-Current and noncurrent accrued and other liabilities:
Contractual obligations-carbon credits	1,673
Significant Unobservable Inputs (Level 3) [Member] | Interest rate contracts [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities