Recently Issued Accounting Pronouncements - Topic 606 (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Apr. 01, 2018	Mar. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		$ 1,374.7	$ 994.2	$ 4,019.7	$ 2,978.5
Cost of Sales	[1]	595.1	387.1	1,908.8	1,172.9
Gross Profit		779.6	607.1	2,110.9	1,805.6
Income tax (benefit) provision		5.8	447.8	(127.9)	440.4
Net income		49.2	$ (251.1)	181.2	$ 108.7
Accounts receivable, net		544.8		544.8		$ 518.1	$ 563.7
Inventories		702.5		702.5		471.1	476.2
Other current assets		194.1		194.1		137.0	119.8
Other assets		106.4		106.4		47.7	71.8
Long-term deferred tax assets		1,717.1		1,717.1		1,656.5	100.2
Accrued liabilities		407.7		407.7		248.1	229.6
Deferred income on shipments to distributors		0.0		0.0		0.0	333.8
Long-term deferred tax liability		830.5		830.5		221.5	205.8
Retained earnings		3,122.4		3,122.4		$ 3,199.0	1,397.3
Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net							(45.6)
Inventories							(5.1)
Other current assets							17.2
Other assets							0.0
Long-term deferred tax assets							(23.1)
Accrued liabilities							18.5
Deferred income on shipments to distributors							(333.8)
Long-term deferred tax liability							16.8
Retained earnings							$ 241.9
Calculated under Revenue Guidance in Effect before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		1,391.4		4,039.0
Cost of Sales		606.6		1,925.9
Gross Profit		784.8		2,113.1
Income tax (benefit) provision		5.2		(128.1)
Net income		55.0		183.6
Accounts receivable, net		578.6		578.6
Inventories		715.0		715.0
Other current assets		157.3		157.3
Other assets		100.9		100.9
Long-term deferred tax assets		1,739.4		1,739.4
Accrued liabilities		387.4		387.4
Deferred income on shipments to distributors		320.1		320.1
Long-term deferred tax liability		813.7		813.7
Retained earnings		2,865.7		2,865.7
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net sales		(16.7)		(19.3)
Cost of Sales		(11.5)		(17.1)
Gross Profit		(5.2)		(2.2)
Income tax (benefit) provision		0.6		0.2
Net income		(5.8)		(2.4)
Accounts receivable, net		(33.8)		(33.8)
Inventories		(12.5)		(12.5)
Other current assets		36.8		36.8
Other assets		5.5		5.5
Long-term deferred tax assets		(22.3)		(22.3)
Accrued liabilities		20.3		20.3
Deferred income on shipments to distributors		(320.1)		(320.1)
Long-term deferred tax liability		16.8		16.8
Retained earnings		$ 256.7		$ 256.7

[1] Includes share-based compensation expense as follows: Cost of sales$3.4 $3.5 $10.9 $10.6Research and development$19.4 $10.9 $53.2 $31.8Selling, general and administrative$16.6 $9.6 $46.1 $27.6Special (income) charges and other, net$0.2 $— $17.3 $—