Reserve for Unpaid Losses and Loss Adjustment Expenses - Reserve for Unpaid Losses and Loss Adjustments (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 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	32,080
Accident and Health Insurance Product Line [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	(73)	$ (20)
Change in Workers Compensation Discount Including Accretion [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	20	16
General Liability [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	28	10
Package business
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	(7)	0
Professional liability
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	8	0
Bond
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	0	(10)
Net Asbestos Reserves [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	0	0
Net Environmental Reserves [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	0	0
Catastrophes [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	(34)	(13)
Uncollectible Reinsurance [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	11	0
Other Insurance Product Line [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	3	5
Scenario, Adjustment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Reinsurance recoverables	90
Less: payments
Reinsurance recoverables	90
Scenario, Adjustment [Member] | Catastrophes [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	123
Scenario, Forecast [Member] | Catastrophes [Member]
Less: payments
Ending liabilities for unpaid losses and loss adjustment expenses, net	850
Property and Casualty, Commercial Insurance Product Line [Member] | Property Insurance Product Line [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	(12)	(6)
Property and Casualty, Commercial Insurance Product Line [Member] | Automobile
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	(10)	20
Property and Casualty, Personal Insurance Product Line [Member] | Property Insurance Product Line [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	(13)	0
Property and Casualty, Personal Insurance Product Line [Member] | Automobile
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	0	0
Property, Liability and Casualty Insurance Product Line [Member]
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	3,772	3,508	$ 3,957	$ 3,488
Beginning liabilities for unpaid losses and loss adjustment expenses, net	19,818	19,057
Provision for unpaid losses and loss adjustment expenses
Current incurral year	3,362	3,563
Prior incurral year development [1]	(79)	2
Total provision for unpaid losses and loss adjustment expenses [2]	3,283	3,565
Less: payments
Current accident year	934	1,009
Prior accident years	2,308	2,292
Total payments	3,242	3,301
Ending liabilities for unpaid losses and loss adjustment expenses, net	19,859	19,321
Reinsurance recoverables	3,772	3,508	3,957	3,488
Ending liabilities for unpaid losses and loss adjustment expenses, gross	23,631	22,829
Group Insurance Policies [Member]
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	235	212	209	208
Beginning liabilities for unpaid losses and loss adjustment expenses, net	8,303	5,564
Provision for unpaid losses and loss adjustment expenses
Current incurral year	2,317	1,319
Prior year's discount accretion	120	101
Prior incurral year development [1]	(217)	(112)
Total provision for unpaid losses and loss adjustment expenses [2]	2,220	1,308
Less: payments
Current accident year	974	542
Prior accident years	1,335	833
Total payments	2,309	1,375
Ending liabilities for unpaid losses and loss adjustment expenses, net	8,214	5,497
Reinsurance recoverables	235	212	$ 209	$ 208
Ending liabilities for unpaid losses and loss adjustment expenses, gross	8,449	5,709
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	85	48
Group Insurance Policies [Member] | Long-term Disability [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	0	65
Group Insurance Policies [Member] | group life term, disability and accident [Member]
Provision for unpaid losses and loss adjustment expenses
Prior incurral year development [1]	$ 65	$ 45