Long-term Incentive Compensation (Narrative) (Details) $ in Millions	3 Months Ended				6 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2021 award_plan	Sep. 30, 2021 USD ($)	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						3 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						1 year 9 months 18 days
Liability classified awards, unrecognized compensation costs						$ 20
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 9 months 18 days
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						1 year 1 month 6 days
Liability classified awards, unrecognized compensation costs						$ 27
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
Liability classified awards, unrecognized compensation costs						$ 24