Pension and Other Postretirement Benefits - Pension Plan Assets (Details) - Pension Benefits [Member] - United States [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Plan assets invested in separate accounts of subsidiaries	$ 3,800
Plan assets invested in funds offered by the buyer of the retirement benefits business	265
Plan assets invested by others	116
Plan assets at fair value	4,151	$ 4,281	$ 3,977
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 2,246	2,031
Target allocation percentages	55.00%
Federal government and agency [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 0	1
Corporate [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,446	1,124
Asset-backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	32	22
Fund Investments {Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	768	884
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 866	1,165
Target allocation percentages	25.00%
Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 506	689
International, including funds and pooled seperate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 360	476
Other Plan Asset Categories [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentages	20.00%
Securities Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 477	457
Real estate, including pooled separate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	250	300
Commercial mortgage loans [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	110	140
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	36	73
Guaranteed Deposit Account Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	107	63
Cash equivalents and other current assets, net [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 59	$ 52