Claim and Claim Adjustment Expense Reserves (Commercial - Workers' Compensation - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	$ 18,701	$ 17,885	$ 17,965	$ 18,070
Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	8,204	7,849
Workers' Compensation | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	3,000
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,854
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2011	1,984
Other	(15)
Liability for unallocated claim adjustment expenses for accident years presented	49
Total net liability for unpaid claim and claim adjustment expenses	3,872
Workers' Compensation | 2011 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	583	581	571	564	$ 522	$ 478	$ 438	$ 358	$ 249	$ 99
Workers' Compensation | 2012 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	538	536	524	509	470	416	342	232	$ 87
Workers' Compensation | 2013 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	414	411	419	417	370	300	213	$ 80
Workers' Compensation | 2014 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	297	290	282	258	215	159	$ 61
Workers' Compensation | 2015 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	243	231	212	180	131	$ 51
Workers' Compensation | 2016 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	219	198	169	129	$ 53
Workers' Compensation | 2017 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	243	207	151	$ 63
Workers' Compensation | 2018 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	229	163	$ 68
Workers' Compensation | 2019 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	169	$ 71
Workers' Compensation | 2020 | Commercial
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 65