Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Specialty - Other Professional and Management Liability (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	$ 4,167	$ 4,089	$ 4,355
Liability for Unpaid Claims and Claims Adjustment Expense, Net	18,249	18,576	18,927	$ 19,117
Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	5,475
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	2,887
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	104
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	72
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,063
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	614	593	585	564	$ 524	$ 472	$ 397	$ 307	$ 162	$ 32
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	741	719	678	641	600	515	376	181	$ 39
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	798	769	719	638	550	358	195	$ 37
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	721	670	630	541	404	203	$ 31
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	726	682	604	502	313	$ 71
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	648	570	398	248	$ 57
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	583	426	240	$ 51
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	375	212	$ 51
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	209	$ 48
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Specialty - Other Professional and Management Liability [Member]
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 60