Investments - Investments With a Continuous Realized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value
Less than 12 months	$ 145	$ 5
12 months or more	0	24
Total	145	29
Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	0
Asset-backed securities
Fair Value
Less than 12 months	26
12 months or more	0
Total	26
Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	0
Corporate bonds
Fair Value
Less than 12 months	72
12 months or more	0
Total	72
Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	0
U.S. government agencies and government- sponsored agencies
Fair Value
Less than 12 months	7
12 months or more	0
Total	7
Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	0
U.S. treasuries
Fair Value
Less than 12 months	31	5
12 months or more	0	24
Total	31	29
Unrealized Losses
Less than 12 months	0	0
12 months or more	0	0
Total	0	$ 0
Exchange Traded Fund
Fair Value
Less than 12 months	1
12 months or more	0
Total	1
Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	0
Other debt securities
Fair Value
Less than 12 months	8
12 months or more	0
Total	8
Unrealized Losses
Less than 12 months	0
12 months or more	0
Total	$ 0