Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Mar. 31, 2018
Available-for-sale securities
Amortized Cost	$ 3,437,145	$ 3,515,826
Gross Unrealized Gains	602	1,404
Gross Unrealized Losses	(47,827)	(40,444)
Estimated Fair Value	3,389,920	3,476,786
Money Market Funds [Member]
Available-for-sale securities
Amortized Cost	458,706	1,291,891
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	458,706	1,291,891
Financial institution securities [Member]
Available-for-sale securities
Amortized Cost	599,832	434,901
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	599,832	434,901
Non-financial institution securities [Member]
Available-for-sale securities
Amortized Cost	615,958	326,219
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(1,413)	(1,376)
Estimated Fair Value	614,546	324,843
U.S. Government and Agency Securities [Member]
Available-for-sale securities
Amortized Cost	421,679	58,913
Gross Unrealized Gains	9	1
Gross Unrealized Losses	(327)	(272)
Estimated Fair Value	421,361	58,642
Foreign Government and Agency Securities [Member]
Available-for-sale securities
Amortized Cost	174,405	179,957
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	174,405	179,957
Mortgage-Backed Securities [Member]
Available-for-sale securities
Amortized Cost	820,305	866,048
Gross Unrealized Gains	580	660
Gross Unrealized Losses	(23,932)	(22,311)
Estimated Fair Value	796,953	844,397
Asset-backed Securities [Member]
Available-for-sale securities
Amortized Cost	90,426	92,751
Gross Unrealized Gains	12	16
Gross Unrealized Losses	(1,410)	(1,378)
Estimated Fair Value	89,028	91,389
Debt Mutual Fund [Member]
Available-for-sale securities
Amortized Cost	101,350	101,350
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(15,717)	(11,680)
Estimated Fair Value	85,633	89,670
Commercial Mortgage Backed Securities [Member]
Available-for-sale securities
Amortized Cost	146,984	156,296
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(3,595)	(3,427)
Estimated Fair Value	143,389	152,870
Equity Securities [Member]
Available-for-sale securities
Amortized Cost	7,500	7,500
Gross Unrealized Gains	0	726
Gross Unrealized Losses	(1,433)	0
Estimated Fair Value	$ 6,067	$ 8,226

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)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized gain 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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