Benefit Plans - Contribution and Benefit Payment Expectations (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012
Contribution And Benefit Payment [Line Items]
2012 expected contributions, top of range	$ 225
2012 expected contributions, bottom of range	$ 125

Bottom end of range of the employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the fiscal year. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions.

No definition available.

Name:	pgn_DefinedBenefitPlanEstimatedEmployerContributionsInFiscalYearBottomRange
Namespace Prefix:	pgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant