Pension and Other Postretirement Benefits - Pension Plan Assets (Details) (Pension Benefits [Member], USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	$ 4,170,000,000		$ 4,089,000,000		$ 3,665,000,000
Plan assets invested in separate accounts of subsidiaries	3,800,000,000
Plan assets invested in funds offered by the buyer of the retirement benefits business.	361,000,000
Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	772,000,000		1,011,000,000
Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2,152,000,000		1,957,000,000
Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,246,000,000		1,121,000,000		1,098,000,000
Fixed Maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,833,000,000		1,796,000,000
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	1,000,000		0
Fixed Maturities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,791,000,000		1,764,000,000
Fixed Maturities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	41,000,000		32,000,000
Federal government and agency [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	2,000,000		2,000,000
Federal government and agency [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,000,000		0
Federal government and agency [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,000,000		2,000,000
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	1,060,000,000		749,000,000
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	1,025,000,000		725,000,000
Corporate [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	35,000,000		24,000,000
Other asset and mortgage-backed securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	24,000,000		23,000,000
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	21,000,000		18,000,000
Other asset and mortgage-backed securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	3,000,000		5,000,000
Fund Investments And Pooled Separate Accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	747,000,000	[1]	1,022,000,000	[1]
Fund Investments And Pooled Separate Accounts [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	[1]	0	[1]
Fund Investments And Pooled Separate Accounts [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	744,000,000	[1]	1,019,000,000	[1]
Fund Investments And Pooled Separate Accounts [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	3,000,000	[1]	3,000,000	[1]
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	1,097,000,000		1,177,000,000
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	771,000,000		1,011,000,000
Equity securities [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	246,000,000		124,000,000
Equity securities [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	80,000,000		42,000,000
Domestic [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	718,000,000		859,000,000
Domestic [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	640,000,000		824,000,000
Domestic [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	5,000,000		0
Domestic [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	73,000,000		35,000,000
International, including funds and pooled seperate accounts [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	379,000,000	[1]	318,000,000	[1]
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	131,000,000	[1]	187,000,000	[1]
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	241,000,000	[1]	124,000,000	[1]
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	7,000,000	[1]	7,000,000	[1]
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	331,000,000	[1]	251,000,000	[1]
Real estate, including pooled separate accounts [Member] | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	[1]	0	[1]
Real estate, including pooled separate accounts [Member] | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0	[1]	0	[1]
Real estate, including pooled separate accounts [Member] | Fair Value Inputs Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	331,000,000	[1]	251,000,000	[1]
Commercial Mortgage Loans [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	110,000,000		88,000,000
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	110,000,000		88,000,000
Securities Partnerships [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	357,000,000		304,000,000
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	357,000,000		304,000,000		328,000,000
Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	283,000,000		360,000,000
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	283,000,000		360,000,000		327,000,000
Guaranteed Deposit Account Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	44,000,000		44,000,000
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	44,000,000		44,000,000		47,000,000
Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	115,000,000		69,000,000
Cash equivalents | Fair Value Inputs Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	0		0
Cash equivalents | Fair Value Inputs Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets at fair value	115,000,000		69,000,000
Cash equivalents | 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.