Investments (Tables)

12 Months Ended

Mar. 31, 2016

Investments [Abstract]

Summary of available-for-sale securities

The following is a summary of available-for-sale securities at March 31, 2016 (amounts in thousands):

	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	468,290	$	439	$	(99	)	$	468,630
Corporate bonds and debt	1,000		—		—			1,000
Marketable equity securities	2,195		8		—			2,203
Total	$	471,485	$	447	$	(99	)	$	471,833

The following is a summary of available-for-sale securities at March 31, 2015 (amounts in thousands):

	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	741,780	$	676	$	(200	)	$	742,256
Municipal bonds	41,552		155		(9		)	41,698
Auction rate securities	9,825		—		—			9,825
Time deposits (1)	506		—		—			506
Corporate bonds and debt	924,818		2,376		(265		)	926,929
Marketable equity securities	1,362		11,804		—			13,166
Total	$	1,719,843	$	15,011	$	(474	)	$	1,734,380

Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block]

The following tables show all investments in an unrealized loss position for which an other-than-temporary impairment has not been recognized and the related gross unrealized losses and fair value, aggregated by investment category and length of time that the individual securities have been in a continuous unrealized loss position (amounts in thousands):

	March 31, 2016
	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Government agency bonds	$	148,562	$	(99	)	$	—	$	—	$	148,562	$	(99	)
Corporate bonds and debt	—		—			1,000		—		1,000		—
Total	$	148,562	$	(99	)	$	1,000	$	—	$	149,562	$	(99	)

	March 31, 2015
	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Government agency bonds	$	162,948	$	(142	)	$	29,942	$	(58	)	$	192,890	$	(200	)
Municipal bonds	13,318		(9		)	—		—			13,318		(9		)
Corporate bonds and debt	163,095		(219		)	19,021		(46		)	182,116		(265		)
Total	$	339,361	$	(370	)	$	48,963	$	(104	)	$	388,324	$	(474	)

Summary of amortized cost and estimated fair value of available-for-sale securities, by maturity

The amortized cost and estimated fair value of the available-for-sale securities at March 31, 2016, by contractual maturity, excluding marketable equity securities of $2.2 million, which have no contractual maturity, are shown below (amounts in thousands). Expected maturities can differ from contractual maturities because the issuers of the securities may have the right to prepay obligations without prepayment penalties, and the Company views its available-for-sale securities as available for current operations.

	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale
Due in one year or less	$	41,078	$	5	$	(5	)	$	41,078
Due after one year and through five years	428,212		434		(94		)	428,552
Due after five years and through ten years	—		—		—			—
Due after ten years	—		—		—			—
Total	$	469,290	$	439	$	(99	)	$	469,630

The amortized cost and estimated fair value of the available-for-sale securities at March 31, 2015, by maturity, excluding marketable equity securities of $13.2 million and corporate debt of $6.2 million, which have no contractual maturity, are shown below (amounts in thousands). 

	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale
Due in one year or less	$	224,531	$	512	$	(34	)	$	225,009
Due after one year and through five years	1,395,685		2,648		(330		)	1,398,003
Due after five years and through ten years	82,250		47		(110		)	82,187
Due after ten years	9,825		—		—			9,825
Total	$	1,712,291	$	3,207	$	(474	)	$	1,715,024