Investments - Available-for-Sale (Details) $ in Millions	Dec. 31, 2019 USD ($) Security	Dec. 31, 2018 USD ($) Security
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 365	$ 509
In a Continuous Loss Position for 12 Months or More	0	694
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months	2	3
In a Continuous Loss Position for 12 Months or More	$ 0	$ 8
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security	239	285
In a Continuous Loss Position for 12 Months or More | Security	0	516
Corporate debt securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 222	$ 509
In a Continuous Loss Position for 12 Months or More	0	412
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months	1	3
In a Continuous Loss Position for 12 Months or More	$ 0	$ 5
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security	167	285
In a Continuous Loss Position for 12 Months or More | Security	0	298
Mortgage-backed securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 143
In a Continuous Loss Position for 12 Months or More	0
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months	1
In a Continuous Loss Position for 12 Months or More	$ 0
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security	72
In a Continuous Loss Position for 12 Months or More | Security	0
Asset-backed securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months		$ 0
In a Continuous Loss Position for 12 Months or More		68
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months		0
In a Continuous Loss Position for 12 Months or More		$ 1
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security		0
In a Continuous Loss Position for 12 Months or More | Security		52
Municipal securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months		$ 0
In a Continuous Loss Position for 12 Months or More		87
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months		0
In a Continuous Loss Position for 12 Months or More		$ 1
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security		0
In a Continuous Loss Position for 12 Months or More | Security		90
GSEs
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months		$ 0
In a Continuous Loss Position for 12 Months or More		127
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months		0
In a Continuous Loss Position for 12 Months or More		$ 1
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security		0
In a Continuous Loss Position for 12 Months or More | Security		76