Investments (Tables)

9 Months Ended

Dec. 31, 2017

Investments [Abstract]

Summary of available-for-sale securities

The following is a summary of available-for-sale securities at December 31, 2017 (amounts in thousands):

	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	815,940	$	—	$	(5,809	)	$	810,131
Municipal bonds - taxable	10,000		—		(57		)	9,943
Corporate bonds and debt	491,851		—		(1,484		)	490,367
Marketable equity securities	707		1,758		—			2,465
Total	$	1,318,498	$	1,758	$	(7,350	)	$	1,312,906

The following is a summary of available-for-sale securities at March 31, 2017 (amounts in thousands):

	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	227,089	$	3	$	(227	)	$	226,865
Municipal bonds - tax-exempt	55,289		—		(10		)	55,279
Municipal bonds - taxable	10,000		43		—			10,043
Corporate bonds and debt	207,888		53		(169		)	207,772
Marketable equity securities	707		879		—			1,586
Total	$	500,973	$	978	$	(406	)	$	501,545

Summary of available-for-sale securities in a continuous unrealized loss position

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 the length of time that the individual securities have been in a continuous unrealized loss position (amounts in thousands):

	December 31, 2017
	Less than 12 Months					12 Months or Greater					Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
Government agency bonds	$	790,325	$	(5,615	)	$	19,806	$	(194	)	$	810,131	$	(5,809	)
Municipal bonds - taxable	9,943		(57		)	—		—			9,943		(57		)
Corporate bonds and debt	447,061		(1,484		)	—		—			447,061		(1,484		)
Total	$	1,247,329	$	(7,156	)	$	19,806	$	(194	)	$	1,267,135	$	(7,350	)

	March 31, 2017
	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Government agency bonds	$	196,875	$	(227	)	$	—	$	—	$	196,875	$	(227	)
Municipal bonds - tax exempt	55,279		(10		)	—		—		55,279		(10		)
Corporate bonds and debt	132,820		(169		)	—		—		132,820		(169		)
Total	$	384,974	$	(406	)	$	—	$	—	$	384,974	$	(406	)

Summary of investments classified by contractual maturity date

The amortized cost and estimated fair value of the available-for-sale securities at December 31, 2017, by contractual maturity, excluding marketable equity securities of $2.5 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	$	425,436	$	—	$	(387	)	$	425,049
Due after one year and through five years	892,355		—		(6,963		)	885,392
Due after five years and through ten years	—		—		—			—
Total	$	1,317,791	$	—	$	(7,350	)	$	1,310,441