Benefit Plans - Rollforward of level 3 plan assets (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in pension plan assets
Fair value of plan assets, beginning of year	$ 6,284	$ 5,650
Fair value of plan assets, end of year	5,299	6,284	$ 5,650
Significant unobservable inputs (Level 3)
Change in pension plan assets
Fair value of plan assets, beginning of year	39	10	18
Actual return on plan assets: Relating to assets sold during the period	0	0	(1)
Actual return on plan assets: Relating to assets still held at the reporting date	3	0	0
Purchases, sales and settlements, net	(5)	29	(12)
Net transfers in and/or (out) of Level 3	(32)	0	5
Fair value of plan assets, end of year	5	39	10
Significant unobservable inputs (Level 3) \| Equity securities
Change in pension plan assets
Fair value of plan assets, beginning of year	29	0	1
Actual return on plan assets: Relating to assets sold during the period	0	0	(1)
Actual return on plan assets: Relating to assets still held at the reporting date	3	0	0
Purchases, sales and settlements, net	0	29	0
Net transfers in and/or (out) of Level 3	(32)	0	0
Fair value of plan assets, end of year	0	29	0
Significant unobservable inputs (Level 3) \| Municipal
Change in pension plan assets
Fair value of plan assets, beginning of year		0	7
Actual return on plan assets: Relating to assets sold during the period			0
Actual return on plan assets: Relating to assets still held at the reporting date			0
Purchases, sales and settlements, net			(7)
Net transfers in and/or (out) of Level 3			0
Fair value of plan assets, end of year			0
Significant unobservable inputs (Level 3) \| Corporate
Change in pension plan assets
Fair value of plan assets, beginning of year	10	10	10
Actual return on plan assets: Relating to assets sold during the period	0	0	0
Actual return on plan assets: Relating to assets still held at the reporting date	0	0	0
Purchases, sales and settlements, net	(5)	0	(5)
Net transfers in and/or (out) of Level 3	0	0	5
Fair value of plan assets, end of year	$ 5	$ 10	$ 10

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets sold.

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held.

+ References

+ Details

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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.

+ References

No definition available.

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- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

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

Amount, measured using unobservable input, of increase (decrease) in plan asset of defined benefit plan from purchase, sale and settlement of trade associated with underlying investment.

+ References

+ Details

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- Definition

Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan, for assets transferred into (out of) Level 3 of fair value hierarchy. Includes, but is not limited to, transfer due to change in observability of significant inputs.

+ References

+ Details

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