Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	$ 468	$ 454
Non-guaranteed separate accounts	6,918	6,653
Subtotal	7,386	7,107
Non-guaranteed separate accounts priced at NAV as a practical expedient	693	732
Total separate account assets	8,079	7,839
Pension Benefits [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Non-guaranteed separate accounts	3,900	3,800
Fair Value Inputs Level 1 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	204	187
Non-guaranteed separate accounts	1,328	1,204
Subtotal	1,532	1,391
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	264	267
Non-guaranteed separate accounts	5,346	5,216
Subtotal	5,610	5,483
Fair Value Inputs Level 3 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	0	0
Non-guaranteed separate accounts	244	233
Subtotal	244	233
Fair Value Inputs Level 3 [Member] | Pension Benefits [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Non-guaranteed separate accounts	200	$ 200
Separate Account Assets [Member]
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Financial assets classified in Level 3, period increase (decrease)