Revenue from Contracts with Customers - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018
Disaggregation of Revenue [Line Items]
Receivables from contracts with customers, included within Receivables	$ 424,000,000	$ 488,000,000
Deferred revenue, included within Other liabilities	3,100,000,000	$ 3,000,000,000
Revenue recognized	268,000,000
Contract cost	2,300,000,000
Amortization expense, contract cost	171,000,000
Premium deficiency charges, non-insurance warranty contract	0
Performance obligations, total	11,900,000,000
2018 [Member]
Disaggregation of Revenue [Line Items]
Performance obligations, total	1,500,000,000
2019 [Member]
Disaggregation of Revenue [Line Items]
Performance obligations, total	$ 1,800,000,000
Minimum [Member] | CNA Financial [Member]
Disaggregation of Revenue [Line Items]
Non-insurance warranty product term	1 month
Minimum [Member] | Diamond Offshore [Member]
Disaggregation of Revenue [Line Items]
Contract term	2 months
Minimum [Member] | Boardwalk Pipeline [Member]
Disaggregation of Revenue [Line Items]
Contract term	1 year
Maximum [Member] | CNA Financial [Member]
Disaggregation of Revenue [Line Items]
Non-insurance warranty product term	10 years
Maximum [Member] | Diamond Offshore [Member]
Disaggregation of Revenue [Line Items]
Contract term	60 months
Maximum [Member] | Boardwalk Pipeline [Member]
Disaggregation of Revenue [Line Items]
Contract term	20 years