Employee Benefits - Change in Plan Assets Measured at Fair Value (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Change in plan assets
Fair value of plan assets at beginning of year	$ 11,060	$ 10,892
Fair value of plan assets at end of year	10,033	11,060
Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	27	11
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		13
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increases for Purchases	(3)	5
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decreases for Sales		(2)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0
Fair value of plan assets at end of year	24	27
High Yield Bonds | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	7	11
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		(1)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increases for Purchases	0	2
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decreases for Sales		(5)
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0
Fair value of plan assets at end of year	7	7
Emerging Market Bonds | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	1	0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increases for Purchases	0	1
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decreases for Sales		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	(1)
Fair value of plan assets at end of year	0	1
Domestic Equity Securities | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	3	0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increases for Purchases	(2)	0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decreases for Sales		3
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	1
Fair value of plan assets at end of year	2	3
Private Debt | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	15	0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		14
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increases for Purchases	0	1
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decreases for Sales		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0
Fair value of plan assets at end of year	15	15
Cash and Cash Equivalents | Fair Value, Inputs, Level 3
Change in plan assets
Fair value of plan assets at beginning of year	1	0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Assets Transferred into (out of) Level 3		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increases for Purchases	(1)	1
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Decreases for Sales		0
Defined Benefit Plan, Plan Assets Level 3 Reconciliation, Increase (Decrease) for Actual Return (Loss) on Plan Assets Still Held	0
Fair value of plan assets at end of year	$ 0	$ 1