Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Catastrophe Claims [Line Items]
Revenues	$ 2,931	$ 2,820	$ 2,766	$ 2,291	$ 2,777	$ 2,685	$ 2,610	$ 2,695	$ 10,808	$ 10,767	$ 10,134
Net income (loss)	$ 387	$ 213	$ 151	$ (61)	$ 273	$ 107	$ 278	$ 342	$ 690	$ 1,000	$ 813
Basic earnings (loss) per share (in usd per share)	$ 1.42	$ 0.79	$ 0.56	$ (0.23)	$ 1.00	$ 0.39	$ 1.03	$ 1.26	$ 2.54	$ 3.68	$ 2.99
Diluted earnings (loss) per share (in usd per share)	$ 1.42	$ 0.79	$ 0.55	$ (0.23)	$ 1.00	$ 0.39	$ 1.02	$ 1.25	$ 2.53	$ 3.67	$ 2.98
Net realized investment gains (losses)				$ 216					$ 45	$ (21)	$ 38
Loss from catastrophes		$ 160	$ 301
Charge related to recognition of active life reserve premium deficiency		$ 74				$ 216
COVID-19
Liability for Catastrophe Claims [Line Items]
Loss from catastrophes			$ 182
Limited Partnership and Common Stock Investments [Member]
Liability for Catastrophe Claims [Line Items]
Loss on limited partnership and common stock investments				$ 125