Pension and Other Postretirement Benefits - Changes in Level 3 Assets (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Pension Benefits [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	$ 4,089	$ 3,665
Actual return on plan assets [Abstract]
Total actual return on plan assets	257	488
Benefits paid	(291)	(262)
Fair value of plan assets, December 31	4,170	4,089
Pension Benefits [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	1,121	1,098
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	101	84
Assets sold during the period	0	7
Total actual return on plan assets	101	91
Purchases, sales, settlements, net	22	(66)
Transfers into (out of) Level 3	2	(2)
Fair value of plan assets, December 31	1,246	1,121
Pension Benefits [Member] \| Fixed Maturities And Equity Securities [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	74	44
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	1	0
Assets sold during the period	0	7
Total actual return on plan assets	1	7
Purchases, sales, settlements, net	44	25
Transfers into (out of) Level 3	2	(2)
Fair value of plan assets, December 31	121	74
Pension Benefits [Member] \| Real Estate And Mortgage Loans [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	339	352
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	41	29
Assets sold during the period	0	0
Total actual return on plan assets	41	29
Purchases, sales, settlements, net	61	(42)
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	441	339
Pension Benefits [Member] \| Securities Partnerships [Member]
Actual return on plan assets [Abstract]
Fair value of plan assets, December 31	357	304
Pension Benefits [Member] \| Securities Partnerships [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	304	328
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	40	16
Assets sold during the period	0	0
Total actual return on plan assets	40	16
Purchases, sales, settlements, net	13	(40)
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	357	304
Pension Benefits [Member] \| Hedge Funds [Member]
Actual return on plan assets [Abstract]
Fair value of plan assets, December 31	283	360
Pension Benefits [Member] \| Hedge Funds [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	360	327
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	17	38
Assets sold during the period	0	0
Total actual return on plan assets	17	38
Purchases, sales, settlements, net	(94)	(5)
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	283	360
Pension Benefits [Member] \| Guaranteed Deposit Account Contract [Member]
Actual return on plan assets [Abstract]
Fair value of plan assets, December 31	44	44
Pension Benefits [Member] \| Guaranteed Deposit Account Contract [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	44	47
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	2	1
Assets sold during the period	0	0
Total actual return on plan assets	2	1
Purchases, sales, settlements, net	(2)	(4)
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	44	44
Other Postretirement Benefits [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	16	20
Actual return on plan assets [Abstract]
Total actual return on plan assets	1	(1)
Benefits paid	(5)	(3)
Fair value of plan assets, December 31	$ 12	$ 16

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)

-URI http://asc.fasb.org/extlink&oid=6504192

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)

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Actual return on plan assets, separately identifying the amount related to assets sold during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date.

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The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Postretirement Benefits

-URI http://asc.fasb.org/extlink&oid=6521376

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

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Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

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- Definition

Amount of increase (decrease) in plan assets from purchases, sales and settlement of plan obligations.

+ References

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- Definition

Transfers in and out of Level 3. For example, transfers due to changes in the observability of significant inputs.

+ References

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-Publisher FASB

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