Long-term Debt - Borrowing Availability (Details) (Credit Agreement [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Propco [Member]
Debt Instrument [Line Items]
Credit agreement, unused borrowing capacity	$ 48.7
Letters of Credit Outstanding, Amount	22.3
Opco [Member]
Debt Instrument [Line Items]
Credit agreement, unused borrowing capacity	184.0
Letters of Credit Outstanding, Amount	$ 16.0

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Subparagraph b

-Article 5

Name:	us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant