Investments - Unrealized Loss Position Maturities (Details) - Debt Securities [Member] - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 704	$ 2,409
Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	11
Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	693
Maturing in Less than one Year [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	21
Maturing in Less than one Year [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1
Maturing in Less than one Year [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	20
Maturing in One Through Five Years [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	43
Maturing in One Through Five Years [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0
Maturing in One Through Five Years [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	43
Maturing After Five Years Through Ten Years [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	88
Maturing After Five Years Through Ten Years [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	6
Maturing After Five Years Through Ten Years [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	82
Maturing in Greater Than Ten Years [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	64
Maturing in Greater Than Ten Years [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4
Maturing in Greater Than Ten Years [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	60
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0
Residential Mortgage Backed Securities [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0
Residential Mortgage Backed Securities [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2
Commercial Mortgage Backed Securities [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0
Commercial Mortgage Backed Securities [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2
Other Asset-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	486
Other Asset-Backed Securities [Member] | Supporting Experience Rated Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0
Other Asset-Backed Securities [Member] | Supporting Remaining Products [Member]
Debt Securities, Available-for-sale [Line Items]
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	$ 486