Investments and Cash - Narrative (Details) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2017 USD ($)	Jun. 30, 2019 USD ($) Security	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Security manager	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Security
Investment [Line Items]
Restricted Cash		$ 4		$ 4		$ 0
Proceeds from sale of fixed-maturity securities available-for-sale		443	$ 183	914	$ 592
Accrued investment income		$ 86		$ 86		$ 91
Number of securities with unrealized losses greater than 10% of book value for 12 months or more | Security		27		27		38
Total unrealized losses for securities having losses greater than 10% of book value for 12 months or more		$ 29		$ 29		$ 43
Assets held-in-trust		279		$ 279		266
Number of outside managers managing investment portfolio | manager				7
Future Equity Investments [Member]
Investment [Line Items]
Long-term purchase commitment, amount	$ 100
Remaining minimum amount committed		85		$ 85
AGL Subsidiaries [Member]
Investment [Line Items]
Assets held-in-trust		$ 1,653		$ 1,653		$ 1,855