Pension and Other Postretirement Benefits - Pension Plan Assets (Details) - Pension Benefits [Member] - United States [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Plan assets invested in separate accounts of subsidiaries	$ 3,900
Plan assets invested in funds offered by the buyer of the retirement benefits business	342
Plan assets at fair value	4,281	$ 3,977	$ 3,981
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 2,031	1,778
Target allocation percentages	50.00%
Federal government and agency [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 1	1
Corporate [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,124	1,125
Asset-backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	22	22
Fund Investments And Pooled Separate Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	884	630
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 1,165	1,031
Target allocation percentages	30.00%
Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 689	681
International, including funds and pooled seperate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 476	350
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	$ 457	424
Real estate, including pooled separate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	300	289
Commercial Mortgage Loans [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	140	129
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	73	196
Guaranteed Deposit Account Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	63	67
Cash equivalents and other current assets, net [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 52	$ 63