Unpaid losses and loss expenses Unpaid losses and loss expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Jan. 14, 2016
Liability for Claims and Claims Adjustment Expense [Abstract]
Gross unpaid losses and loss expenses, beginning of year	$ 60,540	$ 37,303
Reinsurance Recoverable on unpaid losses, beginning of year	(12,708)	(10,741)
Net unpaid loss, beginning of period	47,832	26,562
Acquisition of subsidiaries			$ 0	$ 21,363
Total, Beginning of year	47,832	47,925
Current Year Claims and Claims Adjustment Expense	4,078	3,965
Prior Year Claims and Claims Adjustment Expense	(289)	(291)
Total, Incurred	3,789	3,674
Net loss and loss expenses paid, Current Year	798	586
Net losses and loss expenses Paid, Prior Years	3,109	2,963
Total, Paid	3,907	3,549
Liability For Unpaid Claims And Claims Adjustment Expense Foreign Currency Revaluation And Other	54	29
Net unpaid Loss, end of period	47,768	48,079
Reinsurance Recoverables on unpaid losses, end of year	12,811	12,127
Gross unpaid losses and loss expenses, end of year	$ 60,579	$ 60,206