Investments Summary of Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Summary of available-for-sale securities [Line Items]
Adjusted Cost	$ 1,318,498	$ 500,973
Available-for-sale, gross unrealized gain	1,758	978
Available-for-sale, gross unrealized loss	(7,350)	(406)
Estimated Fair Value	1,312,906	501,545
Government agency bonds
Summary of available-for-sale securities [Line Items]
Adjusted Cost	815,940	227,089
Available-for-sale, gross unrealized gain	0	3
Available-for-sale, gross unrealized loss	(5,809)	(227)
Estimated Fair Value	810,131	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	0	43
Available-for-sale, gross unrealized loss	(57)	0
Estimated Fair Value	9,943	10,043
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Adjusted Cost	491,851	207,888
Available-for-sale, gross unrealized gain	0	53
Available-for-sale, gross unrealized loss	(1,484)	(169)
Estimated Fair Value	490,367	207,772
Marketable equity securities
Summary of available-for-sale securities [Line Items]
Adjusted Cost	707	707
Available-for-sale, gross unrealized gain	1,758	879
Available-for-sale, gross unrealized loss	0	0
Estimated Fair Value	$ 2,465	$ 1,586