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Commitments and Contingencies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Commitments and Contingencies Disclosure [Line Items]
Approximate asserted claims plus accrued interest assessed by Louisiana Department of Revenue	$ 29.9
Additional estimated assessments including interest by Louisiana Department of Revenue	2.5
Minimum potential range contingency loss	0
Maximum potential range contingency loss	$ 32.4

The estimated maximum amount of possible loss from the contingency.

The estimated minimum amount of possible loss from the contingency.