Quarterly Operating Results (Unaudited) - Realized gains and favorable (adverse) prior year development of the liability for losses and LAE (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Realized gains (losses) and favorable (adverse) development on unpaid loss and loss adjustment expenses
Realized Gains (Losses) on Securities and Subsidiaries	$ 51	$ 2	$ (14)	$ (18)	$ (25)	$ (11)	$ (1)	$ (143)	$ 21	$ (180)
Prior Year Development Favorable (Adverse)	$ (10)	$ (22)	$ (28)	$ 28	$ 5	$ (55)	$ 10	$ 7	$ (32)	$ (33)