Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Catastrophe Claims [Line Items]
Revenues	$ 2,403	$ 2,622	$ 2,574	$ 2,535	$ 2,448	$ 2,398	$ 2,366	$ 2,330	$ 10,134	$ 9,542	$ 9,366
Net income	$ (84)	$ 336	$ 270	$ 291	$ 223	$ 144	$ 272	$ 260	$ 813	$ 899	$ 859
Basic earnings (loss) per share (in usd per share)	$ (0.31)	$ 1.24	$ 0.99	$ 1.07	$ 0.82	$ 0.53	$ 1.01	$ 0.96	$ 2.99	$ 3.32	$ 3.18
Diluted earnings (loss) per share (in usd per share)	$ (0.31)	$ 1.23	$ 0.99	$ 1.07	$ 0.82	$ 0.53	$ 1.00	$ 0.96	$ 2.98	$ 3.30	$ 3.17
Net deferred tax asset remeasurement					$ 83
Hurricane Michael and California Wildfires
Liability for Catastrophe Claims [Line Items]
Catastrophe losses net of reinsurance	$ 107
Hurricane Harvey, Irma and Maria
Liability for Catastrophe Claims [Line Items]
Catastrophe losses net of reinsurance						$ 188
Limited Partnership and Common Stock Investments [Member]
Liability for Catastrophe Claims [Line Items]
Loss on limited partnership and common stock investments	$ 109