Pending Acquisition of New Mexico Gas Company - Additional Information (Detail) (USD $)	0 Months Ended			0 Months Ended		6 Months Ended	12 Months Ended			0 Months Ended
	May 25, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 17, 2013 Components of Rate Base [Member]	Dec. 17, 2013 London Interbank Offered Rate (LIBOR) [Member]	Jun. 24, 2013 Bridge Facility [Member]	Dec. 31, 2013 Bridge Facility [Member]	Dec. 31, 2013 Bridge Facility [Member] Minimum [Member]	Dec. 31, 2013 Bridge Facility [Member] Maximum [Member]	Dec. 17, 2013 Letters of Credit [Member] London Interbank Offered Rate (LIBOR) [Member]	Dec. 17, 2013 New Mexico Gas Company [Member]	Jun. 24, 2013 New Mexico Gas Company [Member] Bridge Facility [Member]
Loans At Acquisition Date [Line Items]
Aggregate Purchase Price	$ 950,000,000					$ 950,000,000
Senior Secured Notes at NMGC	200,000,000
Aggregate liability with respect to such indemnification obligations		30,000,000
Aggregate liability with respect to such indemnification obligations deductible		9,250,000
Bank credit facility		675,000,000	675,000,000			1,075,000,000					125,000,000	125,000,000
Maturity period							364 days
Interest rate margin				0.50%	1.00%			0.25%	2.00%
Increase on variable rate for each period that elapses after acquisition closing							0.25%
Period after which company is subject to an increase in variable rate							90 days
Total capitalization ratio							0.70
Normal capitalization ratio							0.65
Credit agreement maturity date											Dec. 17, 2018
Increase commitment under credit facility				$ 75,000,000						$ 40,000,000