Goodwill - Schedule of Changes in Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill, beginning balance	$ 351	$ 374
Impairments		(20)
Other adjustments	(5)	(3)
Goodwill, ending balance	346	351
CNA Financial [Member]
Goodwill [Line Items]
Goodwill, beginning balance	114	117
Other adjustments	(5)	(3)
Goodwill, ending balance	109	114
Diamond Offshore [Member]
Goodwill [Line Items]
Goodwill, beginning balance		20
Impairments		(20)
Boardwalk Pipeline [Member]
Goodwill [Line Items]
Goodwill, beginning balance	237	237
Goodwill, ending balance	$ 237	$ 237