Commitments, Guarantees and Contingent Liabilities - Florida Hurricane Catastrophe Fund and California Earthquake Authority (Details) - USD ($) $ in Millions		12 Months Ended
	Sep. 30, 2017	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Florida Hurricane Catastrophe Fund
Loss Contingencies
Maximum emergency assessment as percent of premiums, first year			6.00%
Emergency assessment maximum percentage of premiums subsequent years			10.00%
Emergency assessment maximum percentage of premiums subsequent years, FL OIR			0.00%
Catastrophe fund, bonds issued		$ 2,000
California Earthquake Authority
Loss Contingencies
CEA capital balance	$ 5,430
Proceeds of revenue bonds	685
CEA existing reinsurance layer	7,510
Current estimated maximum CEA assessment	1,660
CEA capital balance threshold	350
Estimated maximum second CEA assessment	54
CEA threshold earthquake losses	15,330
CEA capital balance to be restored	350
Projected aggregate claim paying capacity	$ 15,330
Share of the CEA (as a percent)				11.20%
Maximum possible assessment			$ 191