Derivatives, Hedges, Financial Instruments and Carbon Credits - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Liabilities-Current and noncurrent accrued and other liabilities:
Total	$ 1,965	$ 2,283
Recurring [Member]
Assets-Supplies, prepaid items and other:
Total	170	53
Liabilities-Current and noncurrent accrued and other liabilities:
Total	1,965	2,283
Commodities contracts [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets	79	11
Carbon credits [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Carbon credits	91	42
Liabilities-Current and noncurrent accrued and other liabilities:
Contractual obligations -carbon credits	91	42
Interest rate contracts [Member] | Recurring [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities	1,874	2,241
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Total	79
Liabilities-Current and noncurrent accrued and other liabilities:
Total
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commodities contracts [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets	79
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Carbon credits [Member] | Recurring [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] | Recurring [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Total
Liabilities-Current and noncurrent accrued and other liabilities:
Total	1,874
Significant Other Observable Inputs (Level 2) [Member] | Commodities contracts [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets
Significant Other Observable Inputs (Level 2) [Member] | Carbon credits [Member] | Recurring [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] | Recurring [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities	1,874
Significant Unobservable Inputs (Level 3) [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Total	91
Liabilities-Current and noncurrent accrued and other liabilities:
Total	91
Significant Unobservable Inputs (Level 3) [Member] | Commodities contracts [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Derivative Assets
Significant Unobservable Inputs (Level 3) [Member] | Carbon credits [Member] | Recurring [Member]
Assets-Supplies, prepaid items and other:
Carbon credits	91
Liabilities-Current and noncurrent accrued and other liabilities:
Contractual obligations -carbon credits	91
Significant Unobservable Inputs (Level 3) [Member] | Interest rate contracts [Member] | Recurring [Member]
Liabilities-Current and noncurrent accrued and other liabilities:
Derivative Financial Instruments, Liabilities