Shareholders' Equity - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Rate of options exercisable per year, commencing one year after grant	20% per year commencing one year after grant
Duration of options expiration, after date of grant	10 years
Total intrinsic value of options exercised	$ 17	$ 46	$ 57
Cash received from the exercise of stock options	14	31	29
Tax benefit related to exercise of stock options	3	8	9
Total compensation expense related to stock incentive plans	20	23	23
Tax benefit related to compensation costs	9	13	13
Cumulative effect of accounting change	6,789	6,269	4,972		$ 5,331
Cumulative effect, period of adoption, adjustment
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative effect of accounting change					$ 4
Accumulated net investment gain (loss) attributable to parent | Cumulative effect, period of adoption, adjustment
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cumulative effect of accounting change	$ 0	0		$ (221)
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted common stock vesting period	4 years
Restricted Common Stock vested during period, fair value	$ 19	$ 11	$ 10
Unrecognized compensation expense related to equity-based awards that have yet to vest	$ 35
Weighted average period of cost expected to be recognized	2 years 6 months
Share-based Payment Arrangement
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures	0	0	0
Common Stock reserved for issuance under stock incentive plans (shares)	3,900,000
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expense related to equity-based awards that have yet to vest	$ 0