Investments, AFS Maturities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	$ 877,933	$ 500,973
Available-for-sale, gross unrealized gain	1,106	978
Available-for-sale, gross unrealized loss	(1,371)	(406)
Estimated Fair Value	877,668	501,545
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	707	707
Available-for-sale, gross unrealized gain	1,032	879
Available-for-sale, gross unrealized loss	0	0
Estimated Fair Value	1,739	$ 1,586
Due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	400,654
Available-for-sale, gross unrealized gain	5
Available-for-sale, gross unrealized loss	(306)
Estimated Fair Value	400,353
Due after one year and through five years
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	476,572
Available-for-sale, gross unrealized gain	69
Available-for-sale, gross unrealized loss	(1,065)
Estimated Fair Value	475,576
Due after five years and through ten years
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	0
Available-for-sale, gross unrealized gain	0
Available-for-sale, gross unrealized loss	0
Estimated Fair Value	0
Due after ten years
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	0
Available-for-sale, gross unrealized gain	0
Available-for-sale, gross unrealized loss	0
Estimated Fair Value	0
Total
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	877,226
Available-for-sale, gross unrealized gain	74
Available-for-sale, gross unrealized loss	(1,371)
Estimated Fair Value	$ 875,929