Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
U.S corporate income tax rate		21.00%	35.00%
Non-cash increase in net income	$ 200		$ 200
Increase in income tax benefit			268
Net deferred tax liabilities partially offset by provisional charge	78		78
Reduction to net income for uncertain tax positions		$ 6
Uncertain income tax positions		116
Reductions for tax positions related to a prior year		48	1	$ 20
Additions for tax positions related to the current year		3	51	4
Income tax (benefit) expense related to penalties		1	(2)	(23)
Net operating loss carryforwards	169	251	169
Valuation allowance	169	175	169
Other deferred tax assets	211	197	211
Federal [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards		83
Net operating loss carryforwards indefinitely		$ 35
Net operating loss carryforwards expiration year		2034 and 2038
Tax credit carryforwards		$ 57
Tax credit carryforwards expiration year		No later than 2021
Foreign [Member]
Income Tax Contingency [Line Items]
Net operating loss carryforwards indefinitely		$ 83
Net operating loss carryforwards expiration year		2021 and 2028
Net operating loss carryforwards in foreign tax jurisdictions		$ 37
Tax credit carryforwards		$ 43
Tax credit carryforwards expiration year		2020 and 2024 to 2027
CNA Financial [Member]
Income Tax Contingency [Line Items]
Equity ownership percentage		80.00%
Diamond Offshore [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits that would affect effective tax rate upon recognition	$ 102	$ 82	$ 102	$ 36
Net operating loss carryforwards		98
Valuation allowance		175
Foreign tax credits		45
Other deferred tax assets		$ 32