Insurance - Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Analysis of changes in the liability for losses and loss adjustment expenses, net of reinsurance
Balance at beginning of period				$ 8,127				$ 7,872	$ 8,127		$ 7,872	$ 6,410
Less reinsurance recoverables, net of allowance				2,201				2,227	2,201		2,227	2,122
Net liability at beginning of period				5,926				5,645	5,926		5,645	4,288
Liability for losses and loss adjustment expenses, period increase (decrease) [Abstract]
Provision for losses and LAE occurring in the current year									2,730		2,662	2,488
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges and other	$ 10	$ 22	$ 28	$ (28)	$ (5)	$ 55	$ (10)	$ (7)	32		33
Total losses and LAE incurred									2,762		2,695	2,494
Payments for losses and LAE of:
Current year									(841)		(828)	(789)
Prior years									(1,512)		(1,575)	(1,340)
Total payments									(2,353)		(2,403)	(2,129)
Reserves of businesses acquired									(40)	[1]	0	1,028	[1]
Foreign currency translation and other									(34)		(11)	(36)
Net liability at end of period	6,261				5,926				6,261		5,926	5,645
Add back reinsurance recoverables, net of allowance	2,302				2,201				2,302		2,201	2,227
Gross unpaid losses and LAE included in the balance sheet	$ 8,563				$ 8,127				8,563		8,127	7,872
Special A&E charges
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges and other									36		67	24
Neon exited lines charge
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges and other									57		0	0
Other
Net increase (decrease) in the provision for claims of prior years:
Provision for claims of prior years related to special A&E charges and other									$ (61)		$ (34)	$ (18)

[1] Reflects the November 2016 reinsurance to close transaction at Neon (discussed below) and the acquisition of Summit in April 2014 (discussed in Note B — “Acquisitions and Sale of Businesses”).