Acquisition of UIL - Schedule of Preliminary Allocation of Purchase Price (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Dec. 16, 2015
Business Acquisition [Line Items]
Goodwill – consideration transferred in excess of fair value assigned	$ 3,113	$ 3,115
UIL Holdings [Member]
Business Acquisition [Line Items]
Current assets, including cash of $48 million			$ 500
Other investments			114
Property, plant and equipment, net			3,552
Regulatory assets			966
Other assets			52
Current liabilities			(493)
Regulatory liabilities			(493)
Non-current debt			(1,878)
Other liabilities			(1,201)
Total net assets acquired at fair value			1,119
Goodwill – consideration transferred in excess of fair value assigned			1,754
Total estimated consideration			$ 2,873