Reserve for Unpaid Losses and Loss Adjustment Expenses Level 4 (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 23,405		$ 23,311
Liability for Claims and Claims Adjustment Expense	27,687		27,605
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	142	$ 131
Personal Auto Liability [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	65
Commercial Lines [Member] | Auto Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	20	9
Property, Liability and Casualty Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverables, Including Reinsurance Premium Paid		2,451
Liability for Claims and Claims Adjustment Expense	21,951	21,823	21,833	$ 21,825
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	2,772	2,835	2,776	2,882
Liability for Unpaid Claims and Claims Adjustment Expense, Net	19,179	18,988	19,057	18,943
Current Year Claims and Claims Adjustment Expense	1,762	1,636
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	12	33
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,774	1,669
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	352	358
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	1,300	1,266
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	1,652	1,624
Property, Liability and Casualty Insurance Product Line [Member] | Property, Liability and Casualty Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	12	33
Property, Liability and Casualty Insurance Product Line [Member] | Other Reserve Re Estimates [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	6	8
Property, Liability and Casualty Insurance Product Line [Member] | Uncollectible Reinsurance [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	0
Property, Liability and Casualty Insurance Product Line [Member] | Catastrophes [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(3)	(7)
Property, Liability and Casualty Insurance Product Line [Member] | Net Environmental Reserves [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	0
Property, Liability and Casualty Insurance Product Line [Member] | Net Asbestos Reserves [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	0
Property, Liability and Casualty Insurance Product Line [Member] | Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	(6)
Property, Liability and Casualty Insurance Product Line [Member] | Surety Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(10)	(6)
Property, Liability and Casualty Insurance Product Line [Member] | Professional Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	(33)
Property, Liability and Casualty Insurance Product Line [Member] | Commercial Property [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	1	(2)
Property, Liability and Casualty Insurance Product Line [Member] | Package Business [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	45
Property, Liability and Casualty Insurance Product Line [Member] | General Liability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	10	32
Property, Liability and Casualty Insurance Product Line [Member] | Change in Workers Compensation Discount Including Accretion [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	8	7
Property, Liability and Casualty Insurance Product Line [Member] | Accident and Health Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(20)	(79)
Group Insurance Policies [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense Prior Years Interest	52	54
Liability for Claims and Claims Adjustment Expense	5,736	5,846	5,772	5,889
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	208	219	208	218
Liability for Unpaid Claims and Claims Adjustment Expense, Net	5,528	5,627	$ 5,564	$ 5,671
Current Year Claims and Claims Adjustment Expense	678	638
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(69)	(53)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	661	639
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	178	174
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	519	509
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	697	683
Group Insurance Policies [Member] | group life term, disability and accident [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	35	20
Group Insurance Policies [Member] | Short-term Disability [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	35	35
Operating Expense [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 0	$ 0