Pension - Fair Value of Pension Assets by Category (Details) - Pension Plan - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	$ 4,623	$ 4,441	$ 4,151
Debt securities
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	2,122	2,407
Federal government securities
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	9	0
Corporate securities
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	1,680	1,906
Mortgage and other asset-backed
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	53	41
Fund investments
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	380	460
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	1,449	1,001
Domestic
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	978	582
International, including funds and pooled separate accounts
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	471	419
Securities partnerships
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	463	531
Real estate funds, including pooled separate accounts
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	219	230
Commercial mortgage loans
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	95	96
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	1	24
Guaranteed deposit account contract
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	98	100
Cash equivalents and other current assets, net
Defined Benefit Plan Disclosure [Line Items]
Pension assets at fair value	$ 176	$ 52