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Summary of Significant Accounting Policies - Additional Information (Detail) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
NPL ownership in ICE percentage	65.00%	65.00%	65.00%
Prepaids and other current assets	$ 107,910,000	$ 107,910,000	$ 107,910,000	$ 96,598,000
Customer advances applied as contributions offsetting construction expenditures	20,000,000	20,000,000	20,000,000
Recognized pretax losses, after tax earnings per share impacts		$ 0.24	$ 0.20
NPL Construction Co [Member]
Recognized pretax profit/losses		18,000,000	15,000,000	5,000,000
Percentage of contract completion	95.00%
Recognized cumulative pretax losses on contract to date basis	10,000,000
Plant materials and operating supplies [Member]
Prepaids and other current assets	$ 25,000,000	$ 25,000,000	$ 25,000,000	$ 21,000,000

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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- Definition

The total of the amounts paid in advance for capitalized costs that will be expensed with the passage of time or the occurrence of a triggering event, and will be charged against earnings within one year or the normal operating cycle, if longer, and the aggregate carrying amount of current assets, as of the balance sheet date, not separately presented elsewhere in the balance sheet. Current assets are expected to be realized or consumed within one year (or the normal operating cycle, if longer).

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