Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Apr. 01, 2017
Available-for-sale securities
Amortized Cost	$ 3,609,721	$ 3,392,034
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	7,480	5,571
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(25,338)	(32,672)
Estimated Fair Value	3,591,863	3,364,933
Money Market Funds [Member]
Available-for-sale securities
Amortized Cost	323,034	298,307
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Estimated Fair Value	323,034	298,307
Financial institution securities [Member]
Available-for-sale securities
Amortized Cost	580,717	348,797
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Estimated Fair Value	580,717	348,797
Non-financial institution securities [Member]
Available-for-sale securities
Amortized Cost	418,879	409,109
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	744	647
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(277)	(496)
Estimated Fair Value	419,346	409,260
Auction Rate Securities [Member]
Available-for-sale securities
Estimated Fair Value	0
U.S. Government and Agency Securities [Member]
Available-for-sale securities
Amortized Cost	95,793	130,749
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	5	8
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(134)	(223)
Estimated Fair Value	95,664	130,534
Foreign Government and Agency Securities [Member]
Available-for-sale securities
Amortized Cost	264,788	322,172
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	(51)
Estimated Fair Value	264,788	322,121
Mortgage-Backed Securities [Member]
Available-for-sale securities
Amortized Cost	1,201,005	1,186,732
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	5,409	3,527
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(9,763)	(14,757)
Estimated Fair Value	1,196,651	1,175,502
Asset-backed Securities [Member]
Available-for-sale securities
Amortized Cost	219,255	220,033
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	480	404
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(478)	(686)
Estimated Fair Value	219,257	219,751
Debt Mutual Fund [Member]
Available-for-sale securities
Amortized Cost	101,350	101,350
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(10,860)	(12,674)
Estimated Fair Value	90,490	88,676
Bank Loans [Member]
Available-for-sale securities
Amortized Cost	157,510	153,281
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	643	839
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(254)	(106)
Estimated Fair Value	157,899	154,014
Commercial Mortgage Backed Securities [Member]
Available-for-sale securities
Amortized Cost	247,390	221,504
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	199	146
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	(3,572)	(3,679)
Estimated Fair Value	$ 244,017	$ 217,971

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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&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
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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