Long-term Incentive Compensation (Narrative) (Details) $ in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2021 USD ($) award_plan	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock Based Compensation 2013 Plan
Long-term Incentive Compensation [Line Items]
Service period	3 years
Stock options
Long-term Incentive Compensation [Line Items]
Vesting period	3 years
Expiration period	7 years
Restricted stock
Long-term Incentive Compensation [Line Items]
Vesting period	4 years
Performance units
Long-term Incentive Compensation [Line Items]
Vesting period	3 years				3 years	3 years	3 years
Weighted average period over which unrecognized cost is recognized, years	2 years 1 month 6 days
Liability classified awards, unrecognized compensation costs	$ 22
Number of award plans granted | award_plan	2
Share-based payment arrangement
Long-term Incentive Compensation [Line Items]
Equity-classified awards, unrecognized compensation cost	$ 1
Weighted average period over which unrecognized cost is recognized, years	1 year
Restricted stock units
Long-term Incentive Compensation [Line Items]
Vesting period	3 years	4 years	4 years	4 years
Weighted average period over which unrecognized cost is recognized, years	2 years 2 months 12 days
Liability classified awards, unrecognized compensation costs	$ 32
Performance cash awards
Long-term Incentive Compensation [Line Items]
Vesting period	4 years				4 years
Weighted average period over which unrecognized cost is recognized, years	3 years 6 months
Liability classified awards, unrecognized compensation costs	$ 30