Goodwill and Other Intangible Assets - Schedule of Changes in Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, beginning balance	$ 659	$ 346
Acquisition	8	310
Other adjustments	(2)	3
Goodwill, ending balance	665	659
Corporate [Member]
Goodwill [Line Items]
Goodwill, beginning balance	310
Acquisition	8	310
Goodwill, ending balance	318	310
CNA Financial [Member]
Goodwill [Line Items]
Goodwill, beginning balance	112	109
Other adjustments	(2)	3
Goodwill, ending balance	110	112
Boardwalk Pipeline [Member]
Goodwill [Line Items]
Goodwill, beginning balance	237	237
Goodwill, ending balance	$ 237	$ 237