Goodwill and Other Intangible Assets - Schedule of Changes in Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill, beginning balance	$ 665	$ 659
Acquisitions	100	8
Other adjustments	2	(2)
Goodwill, ending balance	767	665
CNA Financial [Member]
Goodwill [Line Items]
Goodwill, beginning balance	110	112
Other adjustments	2	(2)
Goodwill, ending balance	112	110
Boardwalk Pipeline [Member]
Goodwill [Line Items]
Goodwill, beginning balance	237	237
Goodwill, ending balance	237	237
Corporate [Member]
Goodwill [Line Items]
Goodwill, beginning balance	318	310
Acquisitions	100	8
Goodwill, ending balance	$ 418	$ 318