Employee Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 3,361	$ 3,041	$ 3,569
Interest cost		129	128	$ 133
Actuarial (gain) loss		499	(438)
Settlement		(257)	(169)
Benefits paid		(51)	(49)
Projected benefit obligation at end of year (a)		3,361	3,041	3,569
Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	3,198	2,943	3,273
Actual return on plan assets		559	(118)
Employer contributions		4	6
Settlement		(257)	(169)
Benefits paid		(51)	(49)
Fair value of plan assets at end of year (b)		3,198	2,943	3,273
Accumulated benefit obligation at end of year (a)					$ 3,361	$ 3,041
Funded status					(163)	(98)
Noncurrent asset					1	1
Current liability					(4)	(4)
Long-term liability					(160)	(95)
Net amounts recognized in consolidated balance sheet					$ (163)	$ (98)
Expected return on plan assets		164	198	189
Remeasurement gain (loss)		(104)	122	(55)
Net periodic pension benefit (cost)		$ (69)	$ 192	$ 1
Discount rate assumption used to calculate benefit obligation (percentage)					3.48%	4.37%
Expected long-term rate of return on plan assets		5.75%	5.75%	6.50%
Discount rate		4.37%	4.24%	3.88%
Pension plan investment assets					$ 2,613	$ 2,339
Accrued investment income and other receivables					1	11
Investments measured at net asset value(c)					584	593
Fair value of plan assets	3,198	$ 3,198	$ 3,273	$ 3,273	3,198	2,943
Pension plan expected future benefit payments, 2020					183
Pension plan expected future benefit payments, 2021					186
Pension plan expected future benefit payments, 2022					188
Pension plan expected future benefit payments, 2023					190
Pension plan expected future benefit payments, 2024					190
Pension plan expected future benefit payments, thereafter					929
Employer contributions to multiemployer plans		9	9	18
Withdrawal liability from multiemployer pension plan					101	104
Level 1
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					4	4
Level 2
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					$ 2,609	$ 2,335
Return-seeking securities
Change in Fair Value of Plan Assets [Roll Forward]
Target Allocation					60.00%	60.00%
Actual Allocation					56.20%	54.60%
Liability-matching securities
Change in Fair Value of Plan Assets [Roll Forward]
Target Allocation					40.00%	40.00%
Actual Allocation					43.70%	45.10%
Other investments
Change in Fair Value of Plan Assets [Roll Forward]
Target Allocation					0.00%	0.00%
Actual Allocation					0.10%	0.30%
Cash
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					$ 4	$ 4
Cash | Level 1
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					4	4
Cash | Level 2
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					0	0
Commingled bond funds(a)
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					1,335	1,270
Commingled bond funds(a) | Level 1
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					0	0
Commingled bond funds(a) | Level 2
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					1,335	1,270
Commingled equity funds(a)
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					1,135	952
Commingled equity funds(a) | Level 1
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					0	0
Commingled equity funds(a) | Level 2
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					1,135	952
Collective trust funds(b)
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					139	113
Collective trust funds(b) | Level 1
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					0	0
Collective trust funds(b) | Level 2
Change in Fair Value of Plan Assets [Roll Forward]
Pension plan investment assets					139	113
Alternative funds(a)
Change in Fair Value of Plan Assets [Roll Forward]
Investments measured at net asset value(c)					271	301
Fixed income funds(b)
Change in Fair Value of Plan Assets [Roll Forward]
Investments measured at net asset value(c)					177	164
Real estate funds(c)
Change in Fair Value of Plan Assets [Roll Forward]
Investments measured at net asset value(c)					136	128
Qualified Plan
Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	3,300	3,000
Projected benefit obligation at end of year (a)		3,300	3,000
Change in Fair Value of Plan Assets [Roll Forward]
Accumulated benefit obligation at end of year (a)					3,326	3,007
Nonqualified Plan
Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at beginning of year	$ 35	34
Projected benefit obligation at end of year (a)		$ 35	34
Change in Fair Value of Plan Assets [Roll Forward]
Accumulated benefit obligation at end of year (a)					$ 35	$ 34
Subsequent Event
Change in Fair Value of Plan Assets [Roll Forward]
Expected long-term rate of return on plan assets	5.00%
Discount rate	3.48%
401(k) Plan
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, maximum annual contributions per employee (percentage)		50.00%
Defined contribution plan, employer matching contribution (percentage)		100.00%
Defined contribution plan, employer matching contribution percent of employees' gross pay (percentage)		6.00%
Defined contribution plan, employer's contributions		$ 303	290	274
Retirement Accumulation Plan
Defined Contribution Plan Disclosure [Line Items]
Defined contribution plan, employer's contributions		$ 152	$ 151	$ 139
Defined contribution plan, employer's contribution percent of employees' eligible pay (percentage)		3.00%