Long-term Incentive Compensation (Narrative) (Details) $ in Millions	3 Months Ended				6 Months Ended	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Jun. 30, 2021 USD ($) award_plan	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					1 year 10 months 24 days
Liability classified awards, unrecognized compensation costs					$ 25
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 4 months 24 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 9 months 18 days
Liability classified awards, unrecognized compensation costs					$ 25
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 3 months 18 days
Liability classified awards, unrecognized compensation costs					$ 27