Pension and Other Postretirement Benefits - Pension Plan Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 3,981	$ 4,170	$ 4,089
Plan assets invested in separate accounts of subsidiaries		3,600
Plan assets invested in funds offered by the buyer of the retirement benefits business.		332
Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		604	772
Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		2,025	2,152
Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1,352	1,246	1,121
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 1,646	1,833
Target allocation percentages		50.00%
Fixed Maturities [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 1	1
Fixed Maturities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1,599	1,791
Fixed Maturities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		46	41
Federal government and agency [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		2	2
Federal government and agency [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1	1
Federal government and agency [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1	1
Federal government and agency [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Corporate [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1,067	1,060
Corporate [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Corporate [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		1,026	1,025
Corporate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		41	35
Other asset and mortgage-backed securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		21	24
Other asset and mortgage-backed securities [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Other asset and mortgage-backed securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		19	21
Other asset and mortgage-backed securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		2	3
Fund Investments And Pooled Separate Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	556	747
Fund Investments And Pooled Separate Accounts [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	0	0
Fund Investments And Pooled Separate Accounts [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	553	744
Fund Investments And Pooled Separate Accounts [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	3	3
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 1,060	1,097
Target allocation percentages		25.00%
Equity securities [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 603	771
Equity securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		364	246
Equity securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		93	80
Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		677	718
Domestic [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		585	640
Domestic [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		6	5
Domestic [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		86	73
International, including funds and pooled seperate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	383	379
International, including funds and pooled seperate accounts [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	18	131
International, including funds and pooled seperate accounts [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	358	241
International, including funds and pooled seperate accounts [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	$ 7	7
Other Plan Asset Categories [Member]
Defined Benefit Plan Disclosure [Line Items]
Target allocation percentages		25.00%
Real estate, including pooled separate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	$ 362	331
Real estate, including pooled separate accounts [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	0	0
Real estate, including pooled separate accounts [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	0	0
Real estate, including pooled separate accounts [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	[1]	362	331
Commercial Mortgage Loans [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		131	110
Commercial Mortgage Loans [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Commercial Mortgage Loans [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Commercial Mortgage Loans [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		131	110
Securities Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		406	357
Securities Partnerships [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Securities Partnerships [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Securities Partnerships [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		406	357	304
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		256	283
Hedge Funds [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Hedge Funds [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Hedge Funds [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		256	283	360
Guaranteed Deposit Account Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		58	44
Guaranteed Deposit Account Contract [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Guaranteed Deposit Account Contract [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Guaranteed Deposit Account Contract [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		58	44	$ 44
Cash equivalents and other current assets, net [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		62	115
Cash equivalents and other current assets, net [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		0	0
Cash equivalents and other current assets, net [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		62	115
Cash equivalents and other current assets, net [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value		$ 0	$ 0

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