Investments Summary of Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018
Summary of available-for-sale securities [Line Items]
Adjusted Cost		$ 1,292,500
Available-for-sale, gross unrealized gain		0
Available-for-sale, gross unrealized loss		0
Estimated Fair Value		1,292,500
Time deposits
Summary of available-for-sale securities [Line Items]
Adjusted Cost	$ 11,300	11,500
Available-for-sale, gross unrealized gain	0	0
Available-for-sale, gross unrealized loss	0	0
Estimated Fair Value	11,300	11,500
Government agency bonds
Summary of available-for-sale securities [Line Items]
Adjusted Cost		723,200
Available-for-sale, gross unrealized gain		0
Available-for-sale, gross unrealized loss		0
Estimated Fair Value		723,200
Municipal bonds - taxable
Summary of available-for-sale securities [Line Items]
Adjusted Cost		14,900
Available-for-sale, gross unrealized gain		0
Available-for-sale, gross unrealized loss		0
Estimated Fair Value		14,900
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Adjusted Cost		542,900
Available-for-sale, gross unrealized gain		0
Available-for-sale, gross unrealized loss		0
Estimated Fair Value		542,900
Marketable equity securities
Summary of available-for-sale securities [Line Items]
Adjusted Cost	$ 3,200	$ 2,800