Pension and Other Postretirement Employee Benefit Plans - Changes In Level 3 Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Increase in investments classified in level 3 of the valuation hierarchy	$ 263	$ 307
Investments classified in level 3 of the valuation hierarchy	25,417	23,366
Unrealized gains	343	401
Offset for settlements	113	85
Level 3
Defined Benefit Plan Disclosure [Line Items]
Investments classified in level 3 of the valuation hierarchy	2,952	$ 2,689	$ 2,400
Transfers related to principal-only mortgage backed securities	$ 33