Investments (Tables)

12 Months Ended

Mar. 31, 2018

Investments [Abstract]

Summary of Available-for-Sale Securities

The following is a summary of available-for-sale securities at March 31, 2018 (amounts in millions):

	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
Government agency bonds	$	723.2	$	—	$	—	$	723.2
Municipal bonds - taxable	14.9		—		—		14.9
Time deposits	11.5		—		—		11.5
Corporate bonds and debt	542.9		—		—		542.9
Marketable equity securities	0.7		2.1		—		2.8
Total	$	1,293.2	$	2.1	$	—	$	1,295.3

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

	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	227.1	$	—	$	(0.2	)	$	226.9
Municipal bonds - tax exempt	55.3		—		—			55.3
Municipal bonds - taxable	10.0		—		—			10.0
Corporate bonds and debt	207.9		0.1		(0.2		)	207.8
Marketable equity securities	0.7		0.9		—			1.6
Total	$	501.0	$	1.0	$	(0.4	)	$	501.6

Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value

The following table 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 during the year ended March 31, 2017 (amounts in millions):

	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	$	196.9	$	(0.2	)	$	—	$	—	$	196.9	$	(0.2	)
Municipal bonds - tax exempt	55.3		—			—		—		55.3		—
Corporate bonds and debt	132.8		(0.2		)	—		—		132.8		(0.2		)
Total	$	385.0	$	(0.4	)	$	—	$	—	$	385.0	$	(0.4	)

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, 2018, by contractual maturity, excluding marketable equity securities of $2.8 million, which have no contractual maturity, are shown below (amounts in millions). 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	$	246.5	$	—	$	—	$	246.5
Due after one year and through five years	1,046.0		—		—		1,046.0
Due after five years and through ten years	—		—		—		—
Due after ten years	—		—		—		—
Total	$	1,292.5	$	—	$	—	$	1,292.5

The amortized cost and estimated fair value of the available-for-sale securities at March 31, 2017, by maturity, excluding marketable equity securities of $1.6 million, which have no contractual maturity, are shown below (amounts in millions). 

	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale
Due in one year or less	$	342.7	$	—	$	(0.2	)	$	342.5
Due after one year and through five years	157.6		0.1		(0.2		)	157.5
Due after five years and through ten years	—		—		—			—
Due after ten years	—		—		—			—
Total	$	500.3	$	0.1	$	(0.4	)	$	500.0