Income Taxes - Deferred tax assets (liabilities) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Deferred tax assets (liabilities)
Deferred revenue	$ 1,458				$ 898				$ 1,458	$ 898
Bad debt reserve	525				662				525	662
Tax credit carry forwards	15,401				591				15,401	591
Accrued expenses and reserves	23,116				14,087				23,116	14,087
Operating lease liabilities	20,428								20,428
Share-based compensation expense	1,929				4,911				1,929	4,911
Unrealized losses	4,630				583				4,630	583
Intangible assets					3,324					3,324
Net operating loss	8,637				14,777				8,637	14,777
Total gross deferred tax assets	76,124				39,833				76,124	39,833
Valuation allowance	(9,644)				(2,492)				(9,644)	(2,492)
Total deferred tax assets	66,480				37,341				66,480	37,341
Depreciable assets	(6,497)				(7,351)				(6,497)	(7,351)
Intangible assets	(13,236)								(13,236)
Operating lease right-of-use assets	(18,829)								(18,829)
Acquisition and other liabilities	(2,961)				(8,890)				(2,961)	(8,890)
Goodwill	(10,799)				(8,154)				(10,799)	(8,154)
Total deferred tax liabilities	(52,322)				(24,395)				(52,322)	(24,395)
Net deferred tax assets/(liabilities)	14,158				12,946				14,158	12,946
Increase in valuation allowance									7,152
Income tax benefit	(19,623)	$ 10,363	$ 4,830	$ 4,739	$ 4,611	$ 10,400	$ (402)	$ 5,864	309	$ 20,473	$ 32,888
Deferred tax charge for disregarded entity election									2,879
Expected refund from income taxes									11,377
Expected foreign tax credits									14,324
Income tax expense included in impact of carry back claim									1,524
Net operating loss carry forwards which begin to expire in next five years	4,246								4,246
Capital loss carryover which begin to expire in 2026	4,391								4,391
Deferred tax assets likely to be realized for which a valuation allowance is not provided	14,393								14,393
Other comprehensive income (loss)
Deferred tax assets (liabilities)
Income tax benefit									(3,778)
ASU 2016-09
Deferred tax assets (liabilities)
Income tax benefit									1,024
United States
Deferred tax assets (liabilities)
Total deferred tax assets	28,218								28,218
Foreign Country
Deferred tax assets (liabilities)
Tax credits which begin to expire in March 2024	15,401								15,401
United Kingdom
Deferred tax assets (liabilities)
Total deferred tax assets	$ 673								$ 673