Quarterly Operating Results (Unaudited) - Favorable (adverse) prior year development of the liability for losses and LAE (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Quarterly Financial Information Disclosure [Abstract]
Prior year claims and claims adjustment expense	$ 8	$ 0	$ 77	$ 42	$ 45	$ 12	$ 41	$ 45	$ 127	$ 143