Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Commercial - Auto (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 - Auto [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	2,220
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	410
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	4
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	10
Liability for Unpaid Claims and Claims Adjustment Expense, Net	424
Commercial - Auto [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	351	350	349	348	$ 340	$ 329	$ 295	$ 250	$ 185	$ 93
Commercial - Auto [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	293	292	291	289	274	244	210	158	$ 83
Commercial - Auto [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	270	270	269	257	229	188	128	$ 72
Commercial - Auto [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	279	274	265	240	197	137	$ 72
Commercial - Auto [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	275	264	241	193	141	$ 78
Commercial - Auto [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	276	253	214	157	$ 77
Commercial - Auto [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	195	164	132	$ 73
Commercial - Auto [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	135	100	$ 63
Commercial - Auto [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	95	$ 52
Commercial - Auto [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	$ 51