Commitments, Guarantees and Contingent Liabilities (Details 2) - USD ($)		12 Months Ended
	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2010	Dec. 31, 2008
Loss Contingencies
Proceeds of revenue bonds	$ 680,000,000
FL Citizens
Loss Contingencies
Assessment rate, percent of deficit or prior year premiums		2.00%
Surcharge on policies, deadline for filing prior to imposing		15 days
Emergency assessment maximum percentage of premiums subsequent years, FL OIR		0.00%
LA Citizens
Loss Contingencies
Assessment rate, percent of deficit or prior year premiums		10.00%
Florida Hurricane Catastrophe Fund
Loss Contingencies
Emergency assessment maximum percentage of premiums subsequent years, FL OIR		0.00%
Maximum emergency assessment as percent of premiums, first year		6.00%
Emergency assessment maximum percentage of premiums subsequent years		10.00%
Catastrophe fund, bonds issued				$ 2,000,000,000	$ 676,000,000	$ 625,000,000
Emergency assessment ordered as percent of premiums from January 1, 2007 through December 31, 2010		1.00%
Emergency assessment ordered as percent of premiums after January 1, 2011		1.30%
California Earthquake Authority
Loss Contingencies
CEA capital balance	5,320,000,000
CEA existing reinsurance layer	4,780,000,000
Current estimated maximum CEA assessment	1,660,000,000
CEA capital balance threshold	350,000,000
Estimated maximum second CEA assessment	128,000,000
CEA threshold earthquake losses	12,570,000,000
CEA capital balance to be restored	350,000,000
Projected aggregate claim paying capacity	12,570,000,000
Share of the CEA (as a percent)			12.10%
Maximum possible assessment		$ 217,000,000
Texas Windstorm Insurance Association
Loss Contingencies
Issuance of securities to participating insurers upon utilization of funds set aside		$ 1,000,000,000
Participation ratio		13.00%
New Jersey Unsatisfied Claim and Judgment Fund
Loss Contingencies
Minimum medical benefits portion of personal injury protection coverage for reimbursement to insurers for policies issued or renewed prior to January 1, 1991		$ 75,000
Minimum medical benefits portion of personal injury protection coverage for reimbursement to insurers for policies issued or renewed from January 1, 1991 to December 31, 2004		75,000
Maximum reimbursement to insurers for medical benefits paid under personal injury protection coverage.		250,000
Liability for insurance assessments		6,700,000
North Carolina Reinsurance Facility
Loss Contingencies
Fund's surplus to cover future losses	261,100,000
North Carolina Joint Underwriters Association
Loss Contingencies
Plan surplus receivable		2,500,000
Funds surplus in member's equity		28,100,000
North Carolina Insurance Underwriting Association
Loss Contingencies
Maximum possible assessment		$ 1,000,000,000
Funds surplus in member's equity	$ 1,300,000,000
Percentage of surcharge on property insurance policy statewide to be remitted to the plan		10.00%
Guaranty Funds
Loss Contingencies
Liability for insurance assessments		$ 4,000,000	$ 13,000,000
Insurance assessments, premium tax offsets		$ 9,000,000	$ 14,000,000