Benefit Plans - Assets of Benefit Plans (Details) (Pension Benefits, USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012
Pension Benefits
Assets Of Benefit Plans [Line Items]
Employer contributions made directly to pension plan assets	$ 18

Amount of employer contributions made directly to pension plan assets.

No definition available.

Name:	pgn_EmployerContributionsDirectlyToPensionPlanAssets
Namespace Prefix:	pgn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration