Reinsurance - Net Effect of Commutations of Ceded and Cancellations of Assumed Reinsurance Contracts (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Increase (decrease) in net unearned premium reserve									$ 64	$ 82	$ 0
Increase (decrease) in net par outstanding									1,457	5,107	28
Commutation gains (losses)	$ 0	$ 1	$ (18)	$ 1	$ 0	$ 255	$ 0	$ 73	$ (16)	$ 328	$ 8