Investments, AFS Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	$ 35,824	$ 471,485
Available-for-sale, gross unrealized gain	2	447
Available-for-sale, gross unrealized loss	(114)	(99)
Estimated Fair Value	35,712	471,833
Marketable equity securities
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	707	2,195
Available-for-sale, gross unrealized gain	0	8
Available-for-sale, gross unrealized loss	0	0
Estimated Fair Value	707	$ 2,203
Due in one year or less
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 one year and through five years
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	35,117
Available-for-sale, gross unrealized gain	2
Available-for-sale, gross unrealized loss	(114)
Estimated Fair Value	35,005
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	35,117
Available-for-sale, gross unrealized gain	2
Available-for-sale, gross unrealized loss	(114)
Estimated Fair Value	$ 35,005