Investments, AFS Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Mar. 31, 2017
Schedule 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
Marketable equity securities
Schedule 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
Due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	425,436
Available-for-sale, gross unrealized gain	0
Available-for-sale, gross unrealized loss	(387)
Estimated Fair Value	425,049
Due after one year and through five years
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	892,355
Available-for-sale, gross unrealized gain	0
Available-for-sale, gross unrealized loss	(6,963)
Estimated Fair Value	885,392
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
Total
Schedule of Available-for-sale Securities [Line Items]
Adjusted Cost	1,317,791
Available-for-sale, gross unrealized gain	0
Available-for-sale, gross unrealized loss	(7,350)
Estimated Fair Value	$ 1,310,441

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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