Financial Instruments (Details 1) - USD ($) $ in Thousands	Sep. 30, 2017	Apr. 01, 2017
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	$ 1,112,532	$ 1,310,037
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(8,462)	(15,101)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	323,037	258,680
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(16,876)	(17,571)
Available-for-Sale Securities, Fair Value, Total	1,435,569	1,568,717
Available-for-Sale Securities, Gross Unrealized Losses, Total	(25,338)	(32,672)
Non-financial institution securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	62,033	68,850
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(210)	(492)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	5,606	1,022
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(67)	(4)
Available-for-Sale Securities, Fair Value, Total	67,639	69,872
Available-for-Sale Securities, Gross Unrealized Losses, Total	(277)	(496)
U.S. Government and Agency Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	34,015	64,895
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(125)	(223)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	5,130	0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(9)	0
Available-for-Sale Securities, Fair Value, Total	39,145	64,895
Available-for-Sale Securities, Gross Unrealized Losses, Total	(134)	(223)
Foreign Government Debt Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value		64,857
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses		(51)
Available-for-Sale Securities, 12 Months or Greater, Fair Value		0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses		0
Available-for-Sale Securities, Fair Value, Total		64,857
Available-for-Sale Securities, Gross Unrealized Losses, Total		(51)
Mortgage-Backed Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	730,974	811,058
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(6,516)	(11,872)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	157,026	139,931
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(3,247)	(2,885)
Available-for-Sale Securities, Fair Value, Total	888,000	950,989
Available-for-Sale Securities, Gross Unrealized Losses, Total	(9,763)	(14,757)
Asset-backed Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	135,560	119,845
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(464)	(651)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	4,757	4,689
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(14)	(35)
Available-for-Sale Securities, Fair Value, Total	140,317	124,534
Available-for-Sale Securities, Gross Unrealized Losses, Total	(478)	(686)
Debt Mutual Fund [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	0	0
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	0	0
Available-for-Sale Securities, 12 Months or Greater, Fair Value	90,490	88,676
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(10,860)	(12,674)
Available-for-Sale Securities, Fair Value, Total	90,490	88,676
Available-for-Sale Securities, Gross Unrealized Losses, Total	(10,860)	(12,674)
Bank Loans [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	30,107	15,139
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(254)	(106)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	0	0
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	0	0
Available-for-Sale Securities, Fair Value, Total	30,107	15,139
Available-for-Sale Securities, Gross Unrealized Losses, Total	(254)	(106)
Commercial Mortgage Backed Securities [Member]
Fair values and gross unrealized losses of the investments
Available-for-Sale Securities, Less Than 12 Months, Fair Value	119,843	165,393
Available-for-Sale Securities, Less Than 12 Months, Gross Unrealized Losses	(893)	(1,706)
Available-for-Sale Securities, 12 Months or Greater, Fair Value	60,028	24,362
Available-for-Sale Securities, 12 Months or Greater, Gross Unrealized Losses	(2,679)	(1,973)
Available-for-Sale Securities, Fair Value, Total	179,871	189,755
Available-for-Sale Securities, Gross Unrealized Losses, Total	$ (3,572)	$ (3,679)