Fair Value Information for Financial Assets and Liabilities, as well as Cost or Amortized Cost (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Current assets:
Cash equivalents (including restricted cash accounts)	$ 54,298		$ 61,649
Marketable Securities			18,521
Derivatives:
Assets, Net, Total	60,955		79,820
Put Option
Derivatives:
Derivative Assets	1,842	[1]
Oil Price Swap
Derivatives:
Derivative Assets	336	[1]
Natural Gas, Price Swap
Derivatives:
Derivative Assets	2,804	[2]
Currency Forward Contracts
Derivatives:
Derivative Assets	1,675	[3]
Derivatives:
Derivatives			(890)	[4]
Cost Or Amortized Cost
Current assets:
Cash equivalents (including restricted cash accounts)	54,298		61,649
Marketable Securities			18,284
Derivatives:
Assets, Net, Total	54,298		79,933
Fair Value, Inputs, Level 1
Current assets:
Cash equivalents (including restricted cash accounts)	54,298		61,649
Marketable Securities			18,521
Derivatives:
Assets, Net, Total	54,298		80,170
Fair Value, Inputs, Level 2
Derivatives:
Assets, Net, Total	6,657		(890)
Fair Value, Inputs, Level 2 | Put Option
Derivatives:
Derivative Assets	1,842	[1]
Fair Value, Inputs, Level 2 | Oil Price Swap
Derivatives:
Derivative Assets	336	[1]
Fair Value, Inputs, Level 2 | Natural Gas, Price Swap
Derivatives:
Derivative Assets	2,804	[2]
Fair Value, Inputs, Level 2 | Currency Forward Contracts
Derivatives:
Derivative Assets	1,675	[3]
Derivatives:
Derivatives			$ (890)	[4]

[1] This amount relates to derivatives which represent swap contract on oil prices, valued primarily based on observable inputs, including forward and spot prices for related commodity indices, and are included within "prepaid expenses and other" in the consolidated balance sheet with the corresponding gain or loss being recognized within "electricity revenues" in the consolidated statement of operations and comprehensive income (loss).

[2] This amount relates to derivatives which represent swap contract on natural gas prices, valued primarily based on observable inputs, including forward and spot prices for related commodity indices, and are included within "prepaid expenses and other" in the consolidated balance sheet with the corresponding gain or loss being recognized within "electricity revenues" in the consolidated statement of operations and comprehensive income (loss).

[3] These amounts relate to derivatives which represent currency forward contracts valued primarily based on observable inputs, including forward and spot prices for currencies, netted against contracted rates and then multiplied against notional amounts, and are included within "prepaid expenses and other" in the consolidated balance sheet with the corresponding gain or loss being recognized within "foreign currency translation and transaction gains (losses)" in the consolidated statement of operations and comprehensive income (loss).

[4] These amounts relate to derivatives which represent currency forward contracts valued primarily based on observable inputs, including forward and spot prices for currencies, netted against contracted rates and then multiplied against notional amounts, and are included within "accounts payable and accrued expenses" in the consolidated balance sheet with the corresponding gain or loss being recognized within "foreign currency translation and transaction gains (losses)" in the consolidated statement of operations and comprehensive income (loss).