Income Taxes - Reconciliation Between U.S. Federal Statutory Tax Rate and Effective Tax Rate (Details)	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Reconciliation Between U.S. Federal Statutory Tax Rate and Effective Tax Rate
Provision for income taxes at U.S. federal statutory tax rate	35.00%	21.00%	21.00%	31.60%
State and local taxes (net of federal tax benefit)		4.00%	4.60%	4.80%
Effect of state tax law changes		(3.90%)	(1.30%)	(0.10%)
Federal tax credits		(3.70%)	(1.00%)	(1.10%)
Tax rate differential from tax loss carryback		(5.60%)	(2.80%)	0.00%
Adjustment for prior year provision to return differences		(1.00%)	(1.40%)	0.40%
Revaluation of federal deferred tax liability from TCJA		0.00%	0.00%	(371.60%)
Other, net		0.00%	(0.50%)	(0.60%)
Effective tax rate		10.80%	18.60%	(336.60%)