Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of year		$ 9,678	$ 8,563
Less reinsurance recoverables, net of allowance		2,957	2,302
Net liability at beginning of year		6,721	6,261
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current period		2,337	2,237
Total losses and LAE incurred		2,206	2,239
Payments for losses and LAE of:
Current year		(569)	(530)
Prior years		(1,313)	(1,272)
Total payments		(1,882)	(1,802)
Reserves of business disposed	[1]	(319)	0
Foreign currency translation and other		(4)	32
Net liability at end of period		6,722	6,730
Add back reinsurance recoverables, net of allowance		2,948	2,833
Gross unpaid losses and LAE included in the balance sheet at end of period		9,670	9,563
Special asbestos and environmental reserve charges
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Net increase (decrease) in the provision for claims of prior years		18	89
Other
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Net increase (decrease) in the provision for claims of prior years		$ (149)	$ (87)

[1] Reflects the reinsurance to close transaction at Neon discussed below.