Investments - AFS (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Mar. 31, 2018
Summary of available-for-sale securities [Line Items]
Adjusted Cost		$ 1,292,500
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		1,292,500
Time deposits
Summary of available-for-sale securities [Line Items]
Adjusted Cost	$ 2,200	11,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	2,200	11,500
Government agency bonds
Summary of available-for-sale securities [Line Items]
Adjusted Cost		723,200
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		723,200
Municipal bonds - taxable
Summary of available-for-sale securities [Line Items]
Adjusted Cost		14,900
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		14,900
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Adjusted Cost		542,900
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Estimated Fair Value		542,900
Marketable equity securities
Summary of available-for-sale securities [Line Items]
Adjusted Cost	$ 2,300	$ 2,800