Fair Value Measurements - Separate Account Assets (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	$ 490		$ 490		$ 454
Non-guaranteed separate accounts	7,131		7,131		6,653
Subtotal	7,621		7,621		7,107
Non-guaranteed separate accounts priced at NAV as a practical expedient	724		724		732
Total separate account assets	8,345		8,345		7,839
Pension Benefits [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Non-guaranteed separate accounts	3,900		3,900		3,800
Fair Value Inputs Level 1 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	209		209		187
Non-guaranteed separate accounts	1,342		1,342		1,204
Subtotal	1,551		1,551		1,391
Fair Value Inputs Level 2 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	281		281		267
Non-guaranteed separate accounts	5,554		5,554		5,216
Subtotal	5,835		5,835		5,483
Fair Value Inputs Level 3 [Member]
Financial assets and financial liabilities carried at fair value [Line Items]
Guaranteed separate accounts	0		0		0
Non-guaranteed separate accounts	235		235		233
Subtotal	235		235		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		$ 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)