Summary of Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Mar. 31, 2016
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 500,973	$ 471,485
Available-for-sale, gross unrealized gains	978	447
Available-for-sale, gross unrealized losses	(406)	(99)
Available-for-sale, estimated fair value	501,545	471,833
Government agency bonds
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	227,089	468,290
Available-for-sale, gross unrealized gains	3	439
Available-for-sale, gross unrealized losses	(227)	(99)
Available-for-sale, estimated fair value	226,865	468,630
Municipal bonds
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	55,289
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	(10)
Available-for-sale, estimated fair value	55,279
Auction rate securities
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	10,000
Available-for-sale, gross unrealized gains	43
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	10,043
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	207,888	1,000
Available-for-sale, gross unrealized gains	53	0
Available-for-sale, gross unrealized losses	(169)	0
Available-for-sale, estimated fair value	207,772	1,000
Equity Securities
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	707	2,195
Available-for-sale, gross unrealized gains	879	8
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	$ 1,586	$ 2,203

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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

+ References

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=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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