Investments Summary of Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Mar. 31, 2017
Summary of available-for-sale securities [Line Items]
Adjusted Cost	$ 1,016,733	$ 500,973
Available-for-sale, gross unrealized gain	2,351	978
Available-for-sale, gross unrealized loss	(1,638)	(406)
Estimated Fair Value	1,017,446	501,545
Government agency bonds
Summary of available-for-sale securities [Line Items]
Adjusted Cost	612,396	227,089
Available-for-sale, gross unrealized gain	12	3
Available-for-sale, gross unrealized loss	(1,456)	(227)
Estimated Fair Value	610,952	226,865
Municipal bonds - tax-exempt
Summary of available-for-sale securities [Line Items]
Adjusted Cost		55,289
Available-for-sale, gross unrealized gain		0
Available-for-sale, gross unrealized loss		(10)
Estimated Fair Value		55,279
Municipal bonds - taxable
Summary of available-for-sale securities [Line Items]
Adjusted Cost	10,000	10,000
Available-for-sale, gross unrealized gain	16	43
Available-for-sale, gross unrealized loss	0	0
Estimated Fair Value	10,016	10,043
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Adjusted Cost	393,630	207,888
Available-for-sale, gross unrealized gain	87	53
Available-for-sale, gross unrealized loss	(182)	(169)
Estimated Fair Value	393,535	207,772
Marketable equity securities
Summary of available-for-sale securities [Line Items]
Adjusted Cost	707	707
Available-for-sale, gross unrealized gain	2,236	879
Available-for-sale, gross unrealized loss	0	0
Estimated Fair Value	$ 2,943	$ 1,586