Selected 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
Quarterly Financial Information Disclosure [Abstract]
Total revenues	$ 1,018	$ 969	$ 935	$ 934	$ 917	$ 875	$ 850	$ 861	$ 3,856	$ 3,503	$ 3,275
Insurance costs	792	748	704	683	692	647	630	641	2,927	2,610	2,466
Operating income	63	68	55	82	42	62	76	71	268	251	217
Net income	$ 48	$ 55	$ 46	$ 63	$ 29	$ 51	$ 58	$ 54	$ 212	$ 192	$ 178
Basic net income per share (in dollars per share)	$ 0.69	$ 0.80	$ 0.65	$ 0.91	$ 0.41	$ 0.73	$ 0.82	$ 0.77	$ 3.04	$ 2.72	$ 2.57
Diluted net income per share (in dollars per share)	$ 0.68	$ 0.78	$ 0.64	$ 0.89	$ 0.40	$ 0.71	$ 0.80	$ 0.75	$ 2.99	$ 2.65	$ 2.49