Income Taxes (Details) - Deferred Tax Asset and Liability - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Nov. 30, 2018	Oct. 31, 2018
Income Tax Disclosure [Abstract]
Deferred tax assets, claims and claim adjustment expense reserves	$ 571	$ 930
Deferred tax assets, unearned premium reserves	503	478
Deferred tax assets, compensation-related liabilities	92	61
Deferred tax assets, other	200	191
Total gross deferred tax assets	1,366	1,660
Less: valuation allowance	8	6
Adjusted gross deferred tax assets	1,358	1,654
Deferred tax liabilities, claims and claim adjustment expense reserve discounting (transition rule)	159	560
Deferred tax liabilities, deferred acquisition costs	397	376
Deferred tax liabilities, investments	152	454
Deferred tax liabilities, internally developed software	92	97
Deferred tax liabilities, depreciation	67	57
Deferred tax liabilities, other	46	40
Total gross deferred tax liabilities	913	1,584
Net deferred tax asset	445	70
Increase in valuation allowance	2
Tax expense recognized resulting from deemed repatriation of foreign earnings (part of net charge to record effect of TCJA)		41
Net charge included in total income tax expense related to TCJA at the date of enactment		$ 129
Estimated additional annual tax payments relating to the discounted reserves transitional adjustment associated with TCJA	$ 23		$ 23	$ 70