Reserve for Unpaid Losses and Loss Adjustment Expenses P&C Liabilities for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	$ 32,287
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, gross	32,213
Workers’ compensation
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	(25)	$ (20)
Change in Workers Compensation Discount Including Accretion [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	10	8
Liability
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	8	10
Package business
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	8	0
Commercial Property [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	(13)	1
Professional liability
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	2	0
Bond
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	0	(10)
Commercial Auto Liability [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	(5)	20
Personal Auto Liability [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	0	0
Homeowners
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	(12)	0
Net Asbestos Reserves [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	0	0
Net Environmental Reserves [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	0	0
Catastrophes [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	(3)	(3)
Uncollectible Reinsurance [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	0	0
Other Reserve Re Estimates [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	(2)	6
Property and Casualty Insurance Products
Product Information [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,608	1,774
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	23,775	22,545
Reinsurance recoverables	3,957	3,488
Beginning liabilities for unpaid losses and loss adjustment expenses, net	19,777	19,179	$ 19,818	$ 19,057
Provision for unpaid losses and loss adjustment expenses
Current accident year	1,640	1,762
Prior accident year development	(32)	12
Less: payments
Current accident year	327	352
Prior accident years	1,322	1,300
Total payments	1,649	1,652
Ending liabilities for unpaid losses and loss adjustment expenses, net	19,777	19,179	19,818	19,057
Reinsurance recoverables	3,938	3,478
Ending liabilities for unpaid losses and loss adjustment expenses, gross	23,715	22,657
Group Insurance Policies [Member]
Product Information [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	41	24
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	1,120	661
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning liabilities for unpaid losses and loss adjustment expenses, gross	8,512	5,772
Reinsurance recoverables	209	208
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,265	5,528	8,303	5,564
Provision for unpaid losses and loss adjustment expenses
Current accident year	1,185	678
Liability for Unpaid Claims and Claims Adjustment Expense Prior Years Interest	62	52
Prior accident year development	(127)	(69)
Less: payments
Current accident year	318	178
Prior accident years	840	519
Total payments	1,158	697
Ending liabilities for unpaid losses and loss adjustment expenses, net	8,265	5,528	$ 8,303	$ 5,564
Reinsurance recoverables	233	208
Ending liabilities for unpaid losses and loss adjustment expenses, gross	8,498	5,736
Group Insurance Policies [Member] | Long-term Disability [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development		35
Group Insurance Policies [Member] | group life term, disability and accident [Member]
Provision for unpaid losses and loss adjustment expenses
Prior accident year development	$ 40	$ 35