Recently Issued Accounting Pronouncements (Details) (Defined pension, Reclassification from accumulated other comprehensive income, USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Defined pension \| Reclassification from accumulated other comprehensive income
Recently Issued Accounting Pronouncements
Amounts reclassified	$ 228,000	$ 185,000

Amount after tax of reclassification adjustment from accumulated other comprehensive (income) loss related to net period benefit cost (credit) for pension and other postretirement defined benefit plans.

No definition available.

Name:	mlp_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansNetOfTax
Namespace Prefix:	mlp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration