Segments of Operations - Narrative (Details) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)
Segment Reporting Information [Line Items]
Number of segments | segment						4
Other income		$ 46		$ 43		$ 172	$ 185
Special charges related to the exit of certain lines of business within subsidiaries						65
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Other income	[1]	3		2		$ 46	$ 61
Property and Casualty Insurance | Real Estate Investment
Segment Reporting Information [Line Items]
Other income			$ 32		$ 51
Other
Segment Reporting Information [Line Items]
Special charges to increase asbestos and environmental reserves		5		12
Write off of deferred debt issuance cost due to the retirement of debt				4
Other Lines
Segment Reporting Information [Line Items]
Special charges related to the exit of certain lines of business within subsidiaries			$ 65
Special charges to increase asbestos and environmental reserves		$ 36		$ 67

[1] Includes pretax income of $32 million (before noncontrolling interest) from the sale of an apartment property in the second quarter of 2016 and $51 million (before noncontrolling interest) from the sale of the Le Pavillon Hotel in the second quarter of 2015.