Fair Value - Separate Accounts Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	$ 4,590	$ 3,884
Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	4,331	3,693
Debt securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	3,816	3,286
Equity securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2	3
Common/collective trusts | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	513	404
Cash and Cash Equivalents
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	259	191
Level 1 | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	1,207	782
Level 1 | Debt securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	1,207	782
Level 1 | Equity securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0	0
Level 1 | Common/collective trusts | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0	0
Level 2 | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	3,124	2,907
Level 2 | Debt securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2,609	2,500
Level 2 | Equity securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	2	3
Level 2 | Common/collective trusts | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	513	404
Level 3 | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0	4
Level 3 | Debt securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0	4
Level 3 | Equity securities | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	0	0
Level 3 | Common/collective trusts | Recurring
Fair Value, Separate Account Investment [Line Items]
Separate accounts assets	$ 0	$ 0