Investments - Available-for-Sale Investments (Details) $ in Millions	Mar. 31, 2020 USD ($) Security	Dec. 31, 2019 USD ($) Security
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 1,106	$ 365
In a Continuous Loss Position for 12 Months or More	0	0
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months	30	2
In a Continuous Loss Position for 12 Months or More	$ 0	$ 0
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security	825	239
In a Continuous Loss Position for 12 Months or More | Security	0	0
Corporate debt securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 805	$ 222
In a Continuous Loss Position for 12 Months or More	0	0
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months	21	1
In a Continuous Loss Position for 12 Months or More	$ 0	$ 0
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security	627	167
In a Continuous Loss Position for 12 Months or More | Security	0	0
Mortgage-backed securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 211	$ 143
In a Continuous Loss Position for 12 Months or More	0	0
Unrealized Losses
In a Continuous Loss Position for Less than 12 Months	8	1
In a Continuous Loss Position for 12 Months or More	$ 0	$ 0
Total Number of Positions
In a Continuous Loss Position for Less than 12 Months | Security	128	72
In a Continuous Loss Position for 12 Months or More | Security	0	0
Asset-backed securities
Estimated Fair Value
In a Continuous Loss Position for Less than 12 Months	$ 90
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	70
In a Continuous Loss Position for 12 Months or More | Security	0