Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Catastrophe Claims [Line Items]
Revenues	$ 2,777	$ 2,685	$ 2,610	$ 2,695	$ 2,403	$ 2,622	$ 2,574	$ 2,535	$ 10,767	$ 10,134	$ 9,542
Net income (loss)	$ 273	$ 107	$ 278	$ 342	$ (84)	$ 336	$ 270	$ 291	$ 1,000	$ 813	$ 899
Basic earnings (loss) per share (in usd per share)	$ 1.00	$ 0.39	$ 1.03	$ 1.26	$ (0.31)	$ 1.24	$ 0.99	$ 1.07	$ 3.68	$ 2.99	$ 3.32
Diluted earnings (loss) per share (in usd per share)	$ 1.00	$ 0.39	$ 1.02	$ 1.25	$ (0.31)	$ 1.23	$ 0.99	$ 1.07	$ 3.67	$ 2.98	$ 3.30
Charge related to recognition of active life reserve premium deficiency		$ 170
Hurricane Michael and California Wildfires
Liability for Catastrophe Claims [Line Items]
Catastrophe losses net of reinsurance					$ 107
Limited Partnership and Common Stock Investments [Member]
Liability for Catastrophe Claims [Line Items]
Loss on limited partnership and common stock investments					$ 109