Acquisition of New Mexico Gas Intermediate - Preliminary Purchase Price Allocation (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013	Sep. 02, 2014 NMGI and NMGC [Member]
Business Acquisition [Line Items]
Current assets				$ 48.7
Property, plant and equipment				618.9
OPEB regulatory asset				6.4
Debt-related regulatory asset				23.9
Goodwill	401.8	0	0	401.8
Deferred tax assets				52.8
Other assets				30.2
Total assets				1,182.7
Current liabilities				(35.0)
Long-term debt fair value adjustment and interest assumed				(22.7)
Cost of removal regulatory liability				(100.6)
Deferred tax liabilities				(60.8)
OPEB liability				(9.8)
Deferred credits and other liabilities				(3.8)
Total liabilities				(232.7)
Total purchase price allocation, excluding cash and working capital adjustments				$ 950.0