Leases - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Leased Assets [Line Items]
Rent expense	$ 97	$ 85	$ 94
Operating leases, Future Minimum Lease Payments, 2017	72
Operating leases, Future Minimum Lease Payments, 2018	$ 59
CNA Financial [Member]
Operating Leased Assets [Line Items]
Expected year for relocation of office space under lease agreement	2018
Operating leases, Future Minimum Lease Payments, 2017	$ 10
Operating leases, Future Minimum Lease Payments, 2018	$ 4