Investments - Unrealized Loss Position (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Supporting Experience Rated Products [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	$ 2
Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	659	2,279
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 704	$ 2,409
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	8	36
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	8	$ 36
Debt Securities [Member] | Supporting Experience Rated Products [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	11
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0
Debt Securities [Member] | Supporting Remaining Products [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	693
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 8
US Government Agencies Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	12	8
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 30	$ 26
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	$ 0
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	26	54
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 39	$ 86
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	$ 1
Debt Security, Corporate, US [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	70	1,399
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 94	$ 1,431
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1	16
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 1	$ 16
Debt Security, Corporate, Non-US [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	39	243
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 47	$ 314
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	3
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	$ 3
Residential Mortgage Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	23	45
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 0	$ 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	0
Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 2
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	$ 0
Other Asset-Backed Securities [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	487	516
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 486	$ 528
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	7	15
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 7	$ 15
Redeemable Preferred Stock [Member]
Available-for-sale Securities, Continuous Unrealized Loss Position [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	1	14
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 6	$ 23
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	0	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	$ 0	$ 1