Investments - Debt and Equity Securities (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Gross Unrealized Gain	$ 45	$ 12
Available-for-sale Securities, Gross Unrealized Loss	0	(2)
Available-for-sale Securities, Fair Value	939	916
Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	15,335	15,119
Available-for-sale Securities, Gross Unrealized Gain	570	172
Available-for-sale Securities, Gross Unrealized Loss	(8)	(36)
Available-for-sale Securities, Fair Value	15,897	15,255
US Government Agencies Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,704	1,662
Available-for-sale Securities, Gross Unrealized Gain	40	26
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Fair Value	1,744	1,688
US States and Political Subdivisions Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,246	2,370
Available-for-sale Securities, Gross Unrealized Gain	71	30
Available-for-sale Securities, Gross Unrealized Loss	0	(1)
Available-for-sale Securities, Fair Value	2,317	2,399
Debt Security, Corporate, US [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	6,777	6,444
Available-for-sale Securities, Gross Unrealized Gain	288	61
Available-for-sale Securities, Gross Unrealized Loss	(1)	(16)
Available-for-sale Securities, Fair Value	7,064	6,489
Debt Security, Corporate, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	2,243	2,355
Available-for-sale Securities, Gross Unrealized Gain	110	31
Available-for-sale Securities, Gross Unrealized Loss	0	(3)
Available-for-sale Securities, Fair Value	2,353	2,383
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	577	567
Available-for-sale Securities, Gross Unrealized Gain	18	10
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Fair Value	595	577
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	608	594
Available-for-sale Securities, Gross Unrealized Gain	29	11
Available-for-sale Securities, Gross Unrealized Loss	0	0
Available-for-sale Securities, Fair Value	637	605
Other Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	1,148	1,097
Available-for-sale Securities, Gross Unrealized Gain	8	3
Available-for-sale Securities, Gross Unrealized Loss	(7)	(15)
Available-for-sale Securities, Fair Value	1,149	1,085
Redeemable Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Amortized Cost Basis	32	30
Available-for-sale Securities, Gross Unrealized Gain	6	0
Available-for-sale Securities, Gross Unrealized Loss	0	(1)
Available-for-sale Securities, Fair Value	$ 38	$ 29