Pension and Other Postretirement Benefits - Changes in Level 3 Assets (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Pension Benefits [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	$ 4,170	$ 4,089
Actual return on plan assets [Abstract]
Total actual return on plan assets	75	257
Benefits paid	(270)	(291)
Fair value of plan assets, December 31	3,981	4,170
Pension Benefits [Member] \| Fair Value Inputs Level 3 [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	1,246	1,121
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	110	101
Assets sold during the period	0	0
Total actual return on plan assets	110	101
Purchases, sales, settlements, net	(11)	22
Transfers into (out of) Level 3	7	2
Fair value of plan assets, December 31	1,352	1,246
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	121	74
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	(3)	1
Assets sold during the period	0	0
Total actual return on plan assets	(3)	1
Purchases, sales, settlements, net	14	44
Transfers into (out of) Level 3	7	2
Fair value of plan assets, December 31	139	121
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	441	339
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	58	41
Assets sold during the period	0	0
Total actual return on plan assets	58	41
Purchases, sales, settlements, net	(6)	61
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	493	441
Pension Benefits [Member] \| Securities Partnerships [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	357
Actual return on plan assets [Abstract]
Fair value of plan assets, December 31	406	357
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	357	304
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	50	40
Assets sold during the period	0	0
Total actual return on plan assets	50	40
Purchases, sales, settlements, net	(1)	13
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	406	357
Pension Benefits [Member] \| Hedge Funds [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	283
Actual return on plan assets [Abstract]
Fair value of plan assets, December 31	256	283
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	283	360
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	4	17
Assets sold during the period	0	0
Total actual return on plan assets	4	17
Purchases, sales, settlements, net	(31)	(94)
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	256	283
Pension Benefits [Member] \| Guaranteed Deposit Account Contract [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	44
Actual return on plan assets [Abstract]
Fair value of plan assets, December 31	58	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	44
Actual return on plan assets [Abstract]
Actual return on plan assets still held at reporting date	1	2
Assets sold during the period	0	0
Total actual return on plan assets	1	2
Purchases, sales, settlements, net	13	(2)
Transfers into (out of) Level 3	0	0
Fair value of plan assets, December 31	58	44
Other Postretirement Benefits [Member]
Change in plan assets [Roll Forward]
Fair value of plan assets, January 1	12	16
Actual return on plan assets [Abstract]
Total actual return on plan assets	(1)	1
Benefits paid	(3)	(5)
Fair value of plan assets, December 31	$ 8	$ 12

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=35743120&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.

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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Reference 5: http://www.xbrl.org/2003/role/presentationRef
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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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.

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

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- Definition

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

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