Restructuring Activities - Additional Information (Detail) (Mid-Continent Organization Restructuring)	12 Months Ended
Dec. 31, 2011 Person
Mid-Continent Organization Restructuring
Restructuring Cost and Reserve [Line Items]
Organization restructuring, termination of employees	40

The number of positions eliminated during the period as a result of restructuring activities.

No definition available.

Name:	us-gaap_RestructuringAndRelatedCostNumberOfPositionsEliminated
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration