Pension Plans and Other Postretirement Benefits - Benefit Obligations and Plan Assets (Details) - Pension Plan - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation:
Benefit obligation, beginning of year	$ 6,239	$ 5,841
Interest cost	168	225	$ 25
Actuarial loss	413	530
Benefit payments	(358)	(357)
Benefit obligation, end of year	6,462	6,239	5,841
Change in plan assets:
Beginning balance	6,395	5,663
Actual return on plan assets	783	1,064
Employer contributions	25	25	12
Benefit payments	(358)	(357)
Ending balance	6,845	6,395	$ 5,663
Funded status	383	156
Assets (liabilities) recognized on the consolidated balance sheet
Non-current assets reflected in other assets	744	494
Current liabilities reflected in accrued expenses	(76)	(25)
Non-current liabilities reflected in other long-term liabilities	(285)	(313)
Net assets	$ 383	$ 156

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.