Restructuring (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended			48 Months Ended
	Jun. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2018
Restructuring Cost and Reserve [Line Items]
Gain on sale of assets		$ 4.4	$ 0.0	$ 0.0
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	$ 40.2	40.2	0.1
Restructuring Charges		1.9	83.2
Payments for Restructuring		(15.1)	(45.9)
Restructuring Reserve, Settled without Cash		0.8	(2.8)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)		0.3	(0.7)
Restructuring Reserve, Ending Balance		$ 28.1	40.2	0.1	$ 28.1
Supply Commitment, Period		10 years
Employee separation costs		$ 1.2	39.1	9.6
Contract exit costs		0.7	44.1	0.7
Contract Termination
Restructuring Cost and Reserve [Line Items]
Restructuring and Related Cost, Incurred Cost		$ 19.5
Restructuring Reserve [Roll Forward]
Contract Termination Fee, Period		5 years
Employee Severance
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	5.4	$ 5.4	0.1
Restructuring Charges		1.2	39.1
Payments for Restructuring		(5.9)	(38.9)
Restructuring Reserve, Settled without Cash		(0.2)	(0.5)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)		0.3	(0.7)
Restructuring Reserve, Ending Balance		0.8	5.4	0.1	0.8
Facility Closing
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Beginning Balance	34.8	34.8	0.0
Restructuring Charges		0.7	44.1
Payments for Restructuring		(9.2)	(7.0)
Restructuring Reserve, Settled without Cash		1.0	(2.3)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)		0.0	0.0
Restructuring Reserve, Ending Balance		27.3	34.8	0.0	27.3
Atmel Corporation
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Accrual Adjustment			6.3
Employee separation costs		1.2
Atmel Corporation | Employee Severance
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Accrual Adjustment			6.3
Atmel Corporation | Facility Closing
Restructuring Reserve [Roll Forward]
Restructuring Reserve, Accrual Adjustment			0.0
Micrel Incorporated
Restructuring Reserve [Roll Forward]
Proceeds from sale of property and assets	10.0
Accrued Liabilities
Restructuring Reserve [Roll Forward]
Current		11.9			11.9
Other Noncurrent Liabilities
Restructuring Reserve [Roll Forward]
Non-current		16.2			16.2
Semiconductor products
Restructuring Reserve [Roll Forward]
Employee separation costs		1.2	39.1	9.6	52.2
Contract exit costs		$ 0.7	$ 44.1	$ 0.7	$ 45.3
San Jose Facility | Micrel Incorporated
Restructuring Cost and Reserve [Line Items]
Gain on sale of assets	4.4
Restructuring Reserve [Roll Forward]
Proceeds from sale of property and assets	$ 10.0