Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Catastrophe Claims [Line Items]
Revenues	$ 2,448	$ 2,398	$ 2,366	$ 2,330	$ 2,390	$ 2,433	$ 2,348	$ 2,195	$ 9,542	$ 9,366	$ 9,101
Net income	$ 223	$ 144	$ 272	$ 260	$ 241	$ 343	$ 209	$ 66	$ 899	$ 859	$ 479
Basic earnings (loss) per share (in usd per share)	$ 0.82	$ 0.53	$ 1.01	$ 0.96	$ 0.89	$ 1.27	$ 0.77	$ 0.25	$ 3.32	$ 3.18	$ 1.77
Diluted earnings (loss) per share (in usd per share)	$ 0.82	$ 0.53	$ 1.00	$ 0.96	$ 0.89	$ 1.26	$ 0.77	$ 0.24	$ 3.30	$ 3.17	$ 1.77
Catastrophe losses net of reinsurance									$ 380	$ 165	$ 141
Net deferred tax asset remeasurement	$ (83)								(83)	$ 0	$ 0
Hurricane Harvey, Irma and Maria
Liability for Catastrophe Claims [Line Items]
Catastrophe losses net of reinsurance		$ 188							$ 256