Nature of Operations and Basis of Presentation - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Gas pipe inventory and operating supplies [Member]	Dec. 31, 2012 Gas pipe inventory and operating supplies [Member]
Gas pipe inventory and operating supplies		$ 24	$ 25
Non-cash construction advances - non-cash investing item	$ 7.9