Pension and Other Postretirement Benefits - Pension Plan Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Plan assets invested in separate accounts of subsidiaries		$ 3,700
Plan assets invested in funds offered by the buyer of the retirement benefits business		276
Plan assets at fair value		3,977	$ 3,981	$ 4,170
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 1,778	1,646
Target allocation percentages		50.00%
Federal government and agency [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 1	2
Corporate [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1,125	1,067
Asset-backed [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		22	21
Fund Investments And Pooled Separate Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		630	556
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 1,031	1,060
Target allocation percentages		25.00%
Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 681	677
International, including funds and pooled seperate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	$ 350	383
Other Plan Asset Categories [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentages		25.00%
Securities Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 424	406
Real estate, including pooled separate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	289	362
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		196	256
Commercial Mortgage Loans [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		129	131
Guaranteed Deposit Account Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		67	58
Cash equivalents and other current assets, net [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 63	$ 62

[1] A pooled separate account has several participating benefit plans and each owns a share of the total pool of investments.