Income Taxes (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019
Income Taxes [Line Items]
Deferred tax liability	$ 605,291	$ 615,904
Valuation allowance	20,238	29,350
Reduction to income tax expense if benefits from net operating loss carryforwards are realized	20,238
Unrecognized tax benefits that would affect the effective tax rate	54,380
Interest and penalties	121	$ 138	$ 0
Accelerated receipt of AMT credit refund amount	869
Residual Deferred Tax Asset [Member]
Income Taxes [Line Items]
Valuation allowance	5,519
Federal [Member]
Income Taxes [Line Items]
Operating loss carryforwards	1,033,208
Federal [Member] | Research and Development Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforwards	35,278
State [Member]
Income Taxes [Line Items]
Operating loss carryforwards	1,518,520
State [Member] | Research and Development Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforwards	2,665
Foreign [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 4,117
Operating loss carryforwards, expiration date	Dec. 31, 2024
State and Foreign [Member]
Income Taxes [Line Items]
Valuation allowance	$ 14,719
Accounting Standards Update 2017-01 [Member]
Income Taxes [Line Items]
Deferred tax liability	$ (28,576)
Minimum [Member] | Federal [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration date	Dec. 31, 2031
Minimum [Member] | Federal [Member] | Research and Development Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforwards, expiration date	Dec. 31, 2038
Minimum [Member] | State [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration date	Dec. 31, 2022
Maximum [Member] | Federal [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration date	Dec. 31, 2037
Maximum [Member] | Federal [Member] | Research and Development Tax Credit Carryforward [Member]
Income Taxes [Line Items]
Tax credit carryforwards, expiration date	Dec. 31, 2041
Maximum [Member] | State [Member]
Income Taxes [Line Items]
Operating loss carryforwards, expiration date	Dec. 31, 2040