Summary of Available for Sale (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Dec. 31, 2014	Mar. 31, 2014
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,742,210	$ 1,677,422
Available-for-sale, gross unrealized gains	34,428	3,462
Available-for-sale, gross unrealized losses	(3,295)	(3,990)
Available-for-sale, estimated fair value	1,773,343us-gaap_AvailableForSaleSecurities	1,676,894us-gaap_AvailableForSaleSecurities
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than Twelve Months	1,042,371	781,501
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Twelve Months or Longer	70,683	1,196
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position	1,113,054	782,697
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than 12 Months	(2,956)	(3,989)
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, 12 Months or Longer	(339)	(1)
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position	(3,295)	(3,990)
Government agency bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	736,615	684,451
Available-for-sale, gross unrealized gains	113	114
Available-for-sale, gross unrealized losses	(1,866)	(3,171)
Available-for-sale, estimated fair value	734,862	681,394
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than Twelve Months	568,466	522,159
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Twelve Months or Longer	64,695	0
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position	633,161	522,159
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than 12 Months	(1,561)	(3,172)
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, 12 Months or Longer	(305)	0
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position	(1,866)	(3,172)
Municipal bonds [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	45,140	41,622
Available-for-sale, gross unrealized gains	74	101
Available-for-sale, gross unrealized losses	(32)	(14)
Available-for-sale, estimated fair value	45,182	41,709
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than Twelve Months	13,533	2,625
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Twelve Months or Longer	0	1,196
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position	13,533	3,821
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than 12 Months	(32)	(13)
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, 12 Months or Longer	0	(1)
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position	(32)	(14)
Auction rate securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	9,825	9,825
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	9,825	9,825
Time Deposits [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	8,699
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	8,699
Corporate bonds and debt [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	938,331	941,524
Available-for-sale, gross unrealized gains	927	3,247
Available-for-sale, gross unrealized losses	(1,397)	(805)
Available-for-sale, estimated fair value	937,861	943,966
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than Twelve Months	460,372	256,717
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position, Twelve Months or Longer	5,988	0
Fair Value of Available-for-sale Securities in a Continuous Unrealized Loss Position	466,360	256,717
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss [Abstract]
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, Less than 12 Months	(1,363)	(804)
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position, 12 Months or Longer	(34)	0
Aggregate Loss, Available-for-sale Securities in a Continuous Unrealized Loss Position	(1,397)	(804)
Equity Securities [Member]
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	3,600
Available-for-sale, gross unrealized gains	33,314
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	$ 36,914

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

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