Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Commercial - General 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 Insurance Contracts, Net Cumulative Paid Claim and 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
Commercial - General Liability [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	4,002
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	2,063
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	1,130
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	55
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,248
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	504	497	492	481	$ 458	$ 413	$ 328	$ 236	$ 130	$ 30
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	567	560	550	528	473	390	261	129	$ 31
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	584	557	532	486	392	270	112	$ 33
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	608	577	530	414	267	139	$ 27
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	534	484	389	253	135	$ 27
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	417	340	233	127	$ 27
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	377	257	135	$ 33
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	245	115	$ 29
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	132	$ 31
Commercial - General Liability [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 34