Fair Value Measurements - Fair Value Option (Details) - USD ($) $ in Millions	1 Months Ended	2 Months Ended	3 Months Ended	5 Months Ended	6 Months Ended
	Jun. 30, 2018	May 31, 2018	Jun. 30, 2017	May 31, 2018	Jun. 30, 2017	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)	$ 0	$ 0	$ 2	$ 0	$ 2
Fair Value, Option, Fixed Maturity Securities	18					$ 32
Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)			3
Equity Securities [Member] | Equity Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)					2
Residential Mortgage Backed Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)	0	0	(1)	0	0
Fair Value, Option, Fixed Maturity Securities	18					$ 32
Fixed Maturities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)	$ 0	$ 0	$ (1)
Fixed Maturities [Member] | Fixed Maturities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)				$ 0	$ 0