Income Taxes - Rate reconciliation (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
Income Taxes
Statutory tax rate (as a percent)									21.00%	21.00%	30.70%
Items which gave rise to differences between the income taxes in the statements of income and the income taxes computed at the U.S. statutory rate
Tax on income before income tax expense at U.S. statutory rate									$ 10,124	$ 8,014	$ 12,865
U.S. state and local taxes (benefit), net of U.S. federal income tax effects									2,478	1,459	(2,800)
Benefit from foreign subsidiaries' tax holidays									(126)	(5,778)	(7,727)
Foreign rate difference									7,668	11,795	(2,215)
Tax rate change									(1,917)	431	9,915
Nondeductible business costs									539	1,032	2,721
Deemed repatriated foreign earnings										(1,628)	17,834
GILTI and BEAT tax									3,367	3,763
Excess stock-based compensation benefits									(1,024)	(3,388)	(1,674)
Nondeductible interest										6,213	6,500
Other adjustments									4,118	(1,157)	(342)
Tax credit carry forwards									(15,565)	548	(1,360)
Income Tax Refundable									(16,940)
Valuation Allowance									7,587	(831)	(829)
Total provision for income taxes	$ (19,623)	$ 10,363	$ 4,830	$ 4,739	$ 4,611	$ 10,400	$ (402)	$ 5,864	$ 309	$ 20,473	$ 32,888