Reserve for Unpaid Losses and Loss Adjustment Expenses Level 4 (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Claims and Claims Adjustment Expense	$ 27,839		$ 27,605
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	220	$ 227
Reinsurance Recoverables, Including Reinsurance Premium Paid	23,265		23,311
Property, Liability and Casualty Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reinsurance Recoverables, Including Reinsurance Premium Paid	2,350
Property, Liability and Casualty Insurance Product Line [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Claims and Claims Adjustment Expense	22,130	21,904	21,833	$ 21,825
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	2,809	2,746	2,776	2,882
Liability for Unpaid Claims and Claims Adjustment Expense, Net	19,321	19,158	19,057	18,943
Reinsurance Recoverables, Including Reinsurance Premium Paid		2,461
Current Year Claims and Claims Adjustment Expense	3,563	3,447
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	2	384
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	3,565	3,831
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	1,009	1,066
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	2,292	2,550
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	3,301	3,616
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)	(83)
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)	16	14
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	66
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	52
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)	(6)	(3)
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] | 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] | Homeowners [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	(5)
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	197
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	71
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)	(13)	(5)
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	(30)
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)	5	8
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)	2	384
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	1
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	140
Group Insurance Policies [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Claims and Claims Adjustment Expense	5,709	5,822	5,772	5,889
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments	212	216	208	218
Liability for Unpaid Claims and Claims Adjustment Expense, Net	5,497	5,606	$ 5,564	$ 5,671
Current Year Claims and Claims Adjustment Expense	1,319	1,287
Liability for Unpaid Claims and Claims Adjustment Expense Prior Years Interest	101	105
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(112)	(98)
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,308	1,294
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	542	523
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	833	836
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	1,375	1,359
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)	45	30
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)	65	65
Operating Expense [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	$ 0	$ 0