Other Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Other Contingencies And Commitments [Line Items]
Current expiration date of agreement	2020
Term of franchise agreement	10 years
Total litigation expense	$ 1,400,000
Total liabilities to tangible net worth minimum times	3.75
Fixed charge coverage ratio minimum times	1.5
Time to cure ratio	30 days
Funds from operations interest coverage ratio minimum times	2
Total debt to capital maximum	65
Total purchase obligations for 2013	69,500,000
Total purchase obligations for 2014 - 2015	99,300,000
Total purchase obligations for 2016 - 2017	82,000,000
Total purchase obligations thereafter	180,200,000
Maximum authorized liability under such guarantees and letters of credit	45,000,000
Aggregate guaranteed amount	29,700,000
Amount of letter of credit to our current primary insurance company	304,000
Guarantees expiring date	June 1,2013
Accruals related to additional sales taxes and gross receipts taxes	82,000	307,000
Delaware division [Member]
Other Contingencies And Commitments [Line Items]
Volume subscribed	30,000
Maximum daily volume subscribed	34,100
Maryland division [Member]
Other Contingencies And Commitments [Line Items]
Volume subscribed	10,000
Maximum daily volume subscribed	15,900
TETLP [Member]
Other Contingencies And Commitments [Line Items]
Amount of letter of credit to our current primary insurance company	2,300,000
Expected TETLP's mainline system expansion	190,000
Number of transportation service contract	2
Letter of credit two [Member]
Other Contingencies And Commitments [Line Items]
Amount of letter of credit to our current primary insurance company	656,000
Issued letter of credit expiry date	Dec. 02, 2013
Letter of credit one [Member]
Other Contingencies And Commitments [Line Items]
Amount of letter of credit to our current primary insurance company	$ 1,000,000