Pension Plans and Other Postretirement Plans - Benefit Obligations and Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation:
Benefit obligation, beginning of year	$ 131	$ 844
Acquired benefit obligations	5,685	0
Interest cost	25	20	$ 27
Actuarial loss (gain)	41	(31)
Benefit payments	(41)	(35)
Settlements	0	(667)
Benefit obligation, end of year	5,841	131	844
Change in plan assets:
Fair value of plan assets, beginning of year	0	624
Fair value of plan assets acquired	5,709	0
Actual return on plan assets	(17)	32
Employer contributions	12	46	25
Benefit payments	(41)	(35)
Settlements	0	(667)
Fair value of plan assets, end of year	5,663	0	$ 624
Funded status	(178)	(131)
Assets (liabilities) recognized on the consolidated balance sheet
Accrued benefit assets reflected in other assets	147	0
Accrued benefit liabilities reflected in accrued expenses	(25)	(21)
Accrued benefit liabilities reflected in other long-term liabilities	(300)	(110)
Net liabilities	$ (178)	$ (131)

Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

Name:	us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.