CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Junior Subordinated Convertible Debentures Due 2037	Common Stock and Additional Paid-in Capital	Common Stock and Additional Paid-in Capital Micrel Incorporated	Common Stock and Additional Paid-in Capital Atmel Corporation	Common Stock Held in Treasury	Accumulated Other Comprehensive Income	Retained Earnings	Parent	Parent Junior Subordinated Convertible Debentures Due 2037	Noncontrolling Interest
Balance at Mar. 31, 2015	$ 2,061.0		$ 999.7			$ (515.7)	$ 11.1	$ 1,549.6	$ 2,044.7		$ 16.3
Balance, common stock shares issued (in shares) at Mar. 31, 2015			218,800,000
Balance, common stock held in treasury (in shares) at Mar. 31, 2015						16,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income from continuing operations	323.9							324.1	324.1		(0.2)
Other comprehensive income (loss)	(14.1)						(14.1)		(14.1)
Purchase of additional interests	(18.1)		$ (1.6)				(0.4)		(2.0)		$ (16.1)
Proceeds from sales of common stock through employee equity incentive plans (shares)			2,500,000
Proceeds from sales of common stock through employee equity incentive plans (amount)	28.7		$ 28.7						28.7
Restricted stock unit and stock appreciation right withholdings (shares)			(500,000)
Restricted stock unit and stock appreciation right withholdings (amount)	(21.7)		$ (21.7)						(21.7)
Treasury stock used for new issuances (shares)			(2,000,000)			(2,000,000)
Treasury stock used for new issuances (amount)			$ (59.4)			$ 59.4
Tax benefit (shortfall) from equity incentive plans	(0.6)		(0.6)						(0.6)
Share-based compensation	73.6		73.6						73.6
Issuance of common stock - acquisitions (shares)				8,600,000
Issuance of common stock - acquisitions (amount)	369.1			$ 369.1					369.1
Non-cash consideration, exchange of employee stock awards (amount)	$ 4.1			$ 4.1					4.1
Purchase of treasury stock (shares)	8,600,000					8,600,000
Purchase of treasury stock (amount)	$ (363.8)					$ (363.8)			(363.8)
Cash dividend	(291.1)							(291.1)	(291.1)
Balance at Mar. 31, 2016	2,151.0		$ 1,391.9			$ (820.1)	(3.4)	1,582.6	2,151.0
Balance, common stock shares issued (in shares) at Mar. 31, 2016			227,400,000
Balance, common stock held in treasury (in shares) at Mar. 31, 2016						23,400,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adoption of ASU 2016-09, cumulative adjustment	49.6		$ 2.0					47.6	49.6
Net income from continuing operations	164.6							164.6	164.6
Other comprehensive income (loss)	(11.0)						(11.0)		(11.0)
Proceeds from sales of common stock through employee equity incentive plans (shares)			4,000,000
Proceeds from sales of common stock through employee equity incentive plans (amount)	42.2		$ 42.2						42.2
Restricted stock unit and stock appreciation right withholdings (shares)			(1,000,000)
Restricted stock unit and stock appreciation right withholdings (amount)	(58.4)		$ (58.4)						(58.4)
Treasury stock used for new issuances (shares)			(3,000,000)			(3,000,000)
Treasury stock used for new issuances (amount)			$ (88.2)			$ 88.2
Share-based compensation	127.3		$ 127.3						127.3
Shares issued to settle convertible debt			12,000,000
Shares issued to settle 2007 junior debt, value	862.7		$ 862.7						862.7
Issuance of common stock - acquisitions (shares)					10,100,000
Issuance of common stock - acquisitions (amount)	486.2				$ 486.2				486.2
Non-cash consideration, exchange of employee stock awards (amount)	7.5				$ 7.5				7.5
Settlement of convertible debt		$ (850.8)	(850.8)							$ (850.8)
Convertible Debt - issuance of debt	615.2		615.2						615.2
Cash dividend	(315.4)							(315.4)	(315.4)
Balance at Mar. 31, 2017	$ 3,270.7		$ 2,537.6			$ (731.9)	(14.4)	1,479.4	3,270.7
Balance, common stock shares issued (in shares) at Mar. 31, 2017	249,463,733		249,500,000
Balance, common stock held in treasury (in shares) at Mar. 31, 2017	20,370,075					20,400,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income from continuing operations	$ 255.4							255.4	255.4
Other comprehensive income (loss)	(3.2)						(3.2)		(3.2)
Proceeds from sales of common stock through employee equity incentive plans (shares)			2,700,000
Proceeds from sales of common stock through employee equity incentive plans (amount)	42.0		$ 42.0						42.0
Restricted stock unit and stock appreciation right withholdings (shares)			(500,000)
Restricted stock unit and stock appreciation right withholdings (amount)	(44.4)		$ (44.4)						(44.4)
Treasury stock used for new issuances (shares)			(2,200,000)			(2,200,000)
Treasury stock used for new issuances (amount)			$ (69.3)			$ 69.3
Share-based compensation	93.1		$ 93.1						93.1
Shares issued to settle convertible debt			3,700,000
Shares issued to settle 2007 junior debt, value	298.5		$ 298.5						298.5
Settlement of convertible debt		$ (337.7)	(337.7)							$ (337.7)
Convertible Debt - issuance of debt	42.9		42.9						42.9
Cash dividend	(337.5)							(337.5)	(337.5)
Balance at Mar. 31, 2018	$ 3,279.8		$ 2,562.7			$ (662.6)	$ (17.6)	$ 1,397.3	$ 3,279.8
Balance, common stock shares issued (in shares) at Mar. 31, 2018	253,232,909		253,200,000
Balance, common stock held in treasury (in shares) at Mar. 31, 2018	18,205,142					18,200,000