Income Taxes (Details) - Deferred Tax Asset and Liability - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
Deferred tax assets, claims and claim adjustment expense reserves	$ 575	$ 551
Deferred tax assets, unearned premium reserves	560	539
Deferred tax assets, compensation-related liabilities	110	97
Deferred tax assets, net operating losses	87	62
Deferred tax assets, other	180	181
Total gross deferred tax assets	1,512	1,430
Less: valuation allowance	21	27
Adjusted gross deferred tax assets	1,491	1,403
Deferred tax liabilities, claims and claim adjustment expense reserve discounting (transition rule)	96	115
Deferred tax liabilities, deferred acquisition costs	445	427
Deferred tax liabilities, investments	1,225	781
Deferred tax liabilities, internally developed software	93	94
Deferred tax liabilities, depreciation	130	70
Deferred tax liabilities, other	60	53
Total gross deferred tax liabilities	2,049	1,540
Net deferred tax liability	$ 558	$ 137