Microchip Technology (MCHP) 10-K 18 May 18 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Mar. 31, 2018	May 09, 2018	Sep. 30, 2017
Document And Entity Information [Abstract]
Entity Registrant Name	MICROCHIP TECHNOLOGY INC
Entity Central Index Key	827,054
Current Fiscal Year End Date	--03-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-K
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Entity Common Stock, Shares Outstanding		235,036,703
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 20,543,563,116

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
ASSETS
Cash and cash equivalents	$ 901.3	$ 908.7
Short-term investments	1,295.3	394.1
Accounts receivable, net	563.7	478.4
Inventories	476.2	417.2
Prepaid expenses	63.9	41.3
Assets held for sale	0	6.4
Other current assets	55.9	58.9
Total current assets	3,356.3	2,305
Property, plant and equipment, net	767.9	683.3
Long-term investments	0	107.5
Goodwill	2,299	2,299
Intangible assets, net	1,662	2,148.1
Long-term deferred tax assets	100.2	68.9
Other assets	71.8	75.1
Total assets	8,257.2	7,686.9
LIABILITIES AND STOCKHOLDERS' EQUITY
Accounts payable	144.1	149.2
Accrued liabilities	229.6	212.5
Deferred income on shipments to distributors	333.8	292.8
Current portion of long-term debt	1,309.9	50
Total current liabilities	2,017.4	704.5
Long-term debt	1,758.4	2,900.5
Long-term income tax payable	754.9	184.9
Long-term deferred tax liability	205.8	409.1
Other long-term liabilities	240.9	217.2
Stockholders' equity:
Preferred stock, $0.001 par value; authorized 5,000,000 shares; no shares issued or outstanding	0	0
Common stock, $0.001 par value; authorized 450,000,000 shares; 253,232,909 shares issued and 235,027,767 shares outstanding at March 31, 2018; 249,463,733 shares issued and 229,093,658 shares outstanding at March 31, 2017	0.2	0.2
Additional paid-in capital	2,562.5	2,537.4
Common stock held in treasury: 18,205,142 shares at March 31, 2018; 20,370,075 shares at March 31, 2017	(662.6)	(731.9)
Accumulated other comprehensive loss	(17.6)	(14.4)
Retained earnings	1,397.3	1,479.4
Total stockholders' equity	3,279.8	3,270.7
Total liabilities and stockholders' equity	$ 8,257.2	$ 7,686.9

CONSOLIDATED BALANCE SHEETS (PA

CONSOLIDATED BALANCE SHEETS (PARENTHETICAL) - $ / shares	Mar. 31, 2018	Mar. 31, 2017
Statement of Financial Position [Abstract]
Common Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Common Stock, Shares Authorized	450,000,000	450,000,000
Common Stock, Shares, Issued	253,232,909	249,463,733
Common Stock, Shares, Outstanding	235,027,767	229,093,658
Preferred Stock, Par or Stated Value Per Share	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized	5,000,000	5,000,000
Preferred Stock, Shares Issued	0	0
Preferred Stock, Shares Outstanding	0	0
Treasury Stock, Shares	18,205,142	20,370,075

CONSOLIDATED STATEMENTS OF INCO

CONSOLIDATED STATEMENTS OF INCOME - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Income Statement [Abstract]
Net sales	$ 3,980.8	$ 3,407.8	$ 2,173.3
Cost of sales	1,560.1	1,650.6	967.8
Gross profit	2,420.7	1,757.2	1,205.5
Operating expenses:
Research and development	529.3	545.3	372.6
Selling, general and administrative	452.1	499.8	301.7
Amortization of acquired intangible assets	485.5	337.7	174.9
Special charges and other, net	17.5	98.6	4
Operating expenses	1,484.4	1,481.4	853.2
Operating income	936.3	275.8	352.3
Losses on equity method investments	(0.2)	(0.2)	(0.3)
Other income (expense):
Interest income	22	3.1	24.4
Interest expense	(199)	(146.3)	(104)
Loss on settlement of convertible debt	(16)	(43.9)	0
Other (loss) income, net	(5.8)	1.3	8.9
Income before income taxes	737.3	89.8	281.3
Income tax provision (benefit)	481.9	(80.8)	(42.6)
Net income from continuing operations	255.4	170.6	323.9
Discontinued operations:
Loss from discontinued operations	0	(7.6)	0
Income tax benefit	0	(1.6)	0
Net loss from discontinued operations	0	(6)	0
Net income from continuing operations	255.4	164.6	323.9
Less: Net loss attributable to noncontrolling interests	0	0	0.2
Net income attributable to Microchip Technology	$ 255.4	$ 164.6	$ 324.1
Basic net income per common share attributable to Microchip Technology stockholders
Net income from continuing operations (in usd per share)	$ 1.10	$ 0.79	$ 1.59
Net loss from discontinued operations (in usd per share)	0	(0.03)	0
Net income attributable to Microchip Technology (in usd per share)	1.10	0.76	1.59
Diluted net income per common share attributable to Microchip Technology stockholders
Net income from continuing operations (in usd per share)	1.03	0.73	1.49
Net loss from discontinued operations (in usd per share)	0	(0.02)	0
Net income attributable to Microchip Technology (in usd per share)	1.03	0.71	1.49
Dividends declared per common share	$ 1.449	$ 1.441	$ 1.433
Basic common shares outstanding	232.9	217.2	203.4
Diluted common shares outstanding	248.9	234.8	217.4

CONSOLIDATED STATEMENTS OF INC5

CONSOLIDATED STATEMENTS OF INCOME (PARENTHETICAL) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Allocated share based compensation expense	$ 93.2	$ 128.1	$ 71.4
Cost of Sales
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Allocated share based compensation expense	13.8	18.7	8.3
Research and Development Expense
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Allocated share based compensation expense	42.5	46.8	32
Selling General And Administrative
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Allocated share based compensation expense	$ 36.9	$ 62.6	$ 31.1

CONSOLIDATED STATEMENTS OF COMP

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 255.4	$ 164.6	$ 323.9
Net loss attributable to noncontrolling interests	0	0	0.2
Net income attributable to Microchip Technology	255.4	164.6	324.1
Available-for-sale securities:
Unrealized holding losses, net of tax effect	(13.6)	(1.5)	(3.2)
Reclassification of realized transactions, net of tax effect	15.2	1.5	(10.9)
Defined benefit plans:
Actuarial losses related to defined benefit pension plans, net of tax benefit of $2.4, $2.2, and $0	(5.6)	(5.3)	0
Reclassification of realized transactions, net of tax effect	0.8	0	0
Change in net foreign currency translation adjustment	0	(5.7)	0
Other comprehensive loss, net of tax effect	(3.2)	(11)	(14.1)
Other comprehensive loss attributable to noncontrolling interests	0	0	0
Net other comprehensive loss	(3.2)	(11)	(14.1)
Comprehensive income	252.2	153.6	309.8
Less: Comprehensive loss attributable to noncontrolling interests	0	0	0.2
Comprehensive income attributable to Microchip Technology	$ 252.2	$ 153.6	$ 310

CONSOLIDATED STATEMENTS OF COM7

CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (PARENTHETICAL) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Other comprehensive (income) loss, pension and other postretirement benefit plans, tax benefit (provision)	$ 2.4	$ 2.2	$ 0

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Cash flows from operating activities:
Net income	$ 255.4	$ 164.6	$ 323.9
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	615.9	469.2	283.2
Deferred income taxes	51.2	(126.9)	(60.4)
Share-based compensation expense related to equity incentive plans	93.2	128.1	71.4
Excess tax benefit from share-based compensation	0	0	(0.8)
Loss on settlement of convertible debt	16	43.9	0
Amortization of debt discount on convertible debt	106.1	56.1	48
Amortization of debt issuance costs	6.6	4.5	4
Losses on equity method investments	0.2	0.2	0.3
Gains on sale of assets	(5.9)	(0.1)	(1)
Loss on write-down of fixed assets	0.1	2.6	0
Impairment of intangible assets	0.5	11.9	0.6
Realized losses (gain) on available-for-sale investments	0	0.1	(13.7)
Realized gain on equity method investment	0	(0.4)	(2.2)
Impairment of available-for-sale investment	15.5	1.4	4
Amortization of premium on available-for-sale investments	0.3	0	9
Changes in operating assets and liabilities, excluding impact of acquisitions:
Increase in accounts receivable	(85.3)	(46.8)	(2.1)
(Increase) decrease in inventories	(59.2)	223.7	48.2
Increase in deferred income on shipments to distributors	41	109.4	17
Decrease in accounts payable and accrued liabilities	(13.9)	(14.8)	(26.4)
Change in other assets and liabilities	14	25.7	0.1
Change in income tax payable	367.9	(2.2)	41.3
Operating cash flows related to discontinued operations	0	9.3	0
Net cash provided by operating activities	1,419.6	1,059.5	744.4
Cash flows from investing activities:
Purchases of available-for-sale investments	(1,594.8)	(500.3)	(1,573.9)
Maturities of available-for-sale investments	786.7	0.4	1,322.7
Sales of available-for-sale investments	0	470.2	1,501.5
Sale of equity method investment	0	1.7	2.7
Acquisition of Atmel, net of cash acquired	0	(2,747.5)	0
Acquisition of Micrel, net of cash acquired	0	0	(343.9)
Purchase of additional controlling interest in ISSC	0	0	(18)
Investments in other assets	(7.1)	(10.2)	(7.1)
Proceeds from sale of assets	10.3	23	14.3
Capital expenditures	(206.8)	(75.3)	(97.9)
Net cash (used in) provided by investing activities	(1,011.7)	(2,838)	800.4
Cash flows from financing activities:
Payments on settlement of convertible debt	(73.4)	(436.2)	0
Proceeds from issuance of 2017 senior debt	0	2,070	0
Proceeds from issuance of 2017 junior debt	0	575	0
Repayments of revolving loan under credit facility	(187)	(2,781)	(1,614.4)
Proceeds from borrowings on revolving loan under credit facility	187	1,537	2,204.5
Deferred financing costs	(1.2)	(36.9)	(2.2)
Payment of cash dividends	(337.5)	(315.4)	(291.1)
Repurchase of common stock	0	0	(363.8)
Proceeds from sale of common stock	42	42.2	28.7
Tax payments related to shares withheld for vested restricted stock units	(44.4)	(58.4)	(21.7)
Capital lease payments	(0.8)	(0.8)	(0.6)
Excess tax benefit from share-based compensation	0	0	0.7
Net cash (used in) provided by financing activities	(415.3)	595.5	(59.9)
Effect of foreign exchange rate changes on cash and cash equivalents	0	(1)	0
Net (decrease) increase in cash and cash equivalents	(7.4)	(1,184)	1,484.9
Cash and cash equivalents at beginning of period	908.7	2,092.7	607.8
Cash and cash equivalents at end of period	$ 901.3	$ 908.7	$ 2,092.7

CONSOLIDATED STATEMENTS OF CAS9

CONSOLIDATED STATEMENTS OF CASH FLOWS NONPRINTING $ in Millions	12 Months Ended
	Mar. 31, 2018USD ($)shares
2007 Junior Subordinated Convertible Notes
Shares issued to settle convertible debt | shares	3,700,000
Extinguishment of debt, amount	$ 143.8
Long-term Debt | 2017 Junior Subordinated Convertible Debt
Notes issued	$ 111.3
Long-term Debt | 2007 Junior Subordinated Convertible Notes
Shares issued to settle convertible debt | shares	3,200,000
Extinguishment of debt, amount	$ 111.3

CONSOLIDATED STATEMENTS OF CHAN

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 CapitalMicrel Incorporated	Common Stock and Additional Paid-in CapitalAtmel Corporation	Common Stock Held in Treasury	Accumulated Other Comprehensive Income	Retained Earnings	Parent	ParentJunior Subordinated Convertible Debentures Due 2037	Noncontrolling Interest
Balance at Mar. 31, 2015	$ 2,061		$ 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)		$ (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		$ 1,391.9			$ (820.1)	(3.4)	1,582.6	2,151
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					47.6	49.6
Net income from continuing operations	164.6							164.6	164.6
Other comprehensive income (loss)	(11)						(11)		(11)
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		$ 42						42
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

Significant Accounting Policies

Significant Accounting Policies	12 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Significant Accounting Policies	Significant Accounting Policies Nature of Business Microchip Technology Incorporated ("Microchip" or the "Company") develops, manufactures and sells specialized semiconductor products used by its customers for a wide variety of embedded control applications. Microchip's product portfolio comprises general purpose and specialized 8-bit, 16-bit, and 32-bit microcontrollers, a broad spectrum of high-performance linear, mixed-signal, power management, thermal management, radio frequency (RF), timing, safety, security, wired connectivity and wireless connectivity devices, as well as serial Electrically Erasable Programmable Read Only Memory (EEPROMs), Serial Flash memories, Parallel Flash memories and serial Static Random Access Memory (SRAM) memories. Microchip also licenses Flash-IP solutions that are incorporated in a broad range of products. Pending Acquisition On March 1, 2018, the Company entered into a definitive agreement to acquire Microsemi Corporation for $68.78 per share in cash. The Company plans to finance the transaction which is expected to close in late May/early June 2018 with a combination of existing cash balances, its line of credit and new debt. The acquisition price represents a total equity value of approximately $8.35 billion , and a total enterprise value of about $10.15 billion , after accounting for Microsemi’s cash and investments, net of debt, on its balance sheet at December 31, 2017. Microsemi is headquartered in Aliso Viejo, California, and has approximately 4,800 employees globally. See Note 23, Subsequent Events, for additional details regarding this acquisition. Principles of Consolidation The Company prepares its consolidated financial statements in accordance with generally accepted accounting principles in the United States of America (US GAAP). The consolidated financial statements include the accounts of Microchip and its majority-owned and controlled subsidiaries. All of the Company's subsidiaries are included in the consolidated financial statements. All significant intercompany accounts and transactions have been eliminated in consolidation. All dollar amounts in the financial statements and tables in these notes, except per share amounts, are stated in millions of U.S. dollars unless otherwise noted. Revenue Recognition (prior to the adoption of ASC 606) The Company recognizes revenue when the earnings process is complete, as evidenced by an agreement with the customer, transfer of title has occurred, the pricing is fixed or determinable and collectability is reasonably assured. The Company recognizes revenue from product sales to original equipment manufacturers (OEMs) upon shipment and records reserves for estimated customer returns. Distributors worldwide generally have broad price protection and product return rights which prevent the sales pricing from being fixed or determinable at the time of the Company's shipment to the distributors. Therefore, revenue recognition is deferred until the pricing uncertainty is resolved, which generally occurs when the distributor sells the product to their customer. At the time of shipment to these distributors, the Company records a trade receivable for the selling price as there is a legally enforceable right to payment, relieves inventory for the carrying value of goods shipped since legal title has passed to the distributor, and records the gross margin in deferred income on shipments to distributors on its consolidated balance sheets. In connection with its acquisitions of Atmel and Micrel, the Company acquired certain distributor relationships where revenue was recognized upon shipment to the distributors based on certain contractual terms or prevailing business practices that resulted in the price being fixed and determinable at such time. Following an acquisition, the Company undertakes efforts to align the contract terms and business practices of the acquired entity with its own. Once these efforts are complete, the related revenue recognition is changed. With respect to such distributor relationships acquired in the Atmel acquisition, as of October 1, 2016, these business practices were conformed to those of the Company’s other distributors, which beginning in October 2016 resulted in the deferral of revenue recognition until the distributor sells the product to their customers. With respect to such distributor relationships acquired in the Micrel acquisition, in the December 2015 quarter, these distributor contracts were changed to be consistent with those of the Company’s other distributors which resulted in the deferral of revenue recognition under such contracts until the distributor sells the product to their customers. Deferred income on shipments to distributors effectively represents gross margin on the sale to the distributor at the initial shipment date; however, the amount of gross margin recognized by the Company in future periods will be less than the deferred margin as a result of credits granted to distributors on specifically identified products and customers to allow the distributors to earn a competitive gross margin on the sale of the Company's products to their end customers and price protection concessions related to market pricing conditions. The Company sells the majority of the items in its product catalog to its distributors worldwide at a uniform list price. However, distributors resell the Company's products to end customers at a broad range of individually negotiated price points. The majority of the Company's distributors' resales require a reduction from the original list price paid. Often, under these circumstances, the Company remits back to the distributor a portion of their original purchase price after the resale transaction is completed in the form of a credit against the distributors' outstanding accounts receivable balance. The credits are on a per unit basis and are not given to the distributor until they provide information regarding the sale to their end customer. The price reductions vary significantly based on the customer, product, quantity ordered, geographic location and other factors and discounts to a price less than the Company's cost have historically been rare. The effect of granting these credits establishes the net selling price from the Company to its distributors for the product and results in the net revenue recognized by the Company when the product is sold by the distributors to their end customers. Thus, a portion of the "deferred income on shipments to distributors" balance represents the amount of distributors' original purchase price that will be credited back to the distributors in the future. The Company does not reduce deferred income on shipments to distributors or accounts receivable by anticipated future price concessions; rather, price concessions are recorded against deferred income on shipments to distributors when incurred, which is generally at the time the distributor sells the product. At March 31, 2018 , the Company had approximately $479.6 million of deferred revenue and $145.8 million in deferred cost of sales recognized as $ 333.8 million of deferred income on shipments to distributors. At March 31, 2017 , the Company had approximately $418.0 million of deferred revenue and $125.2 million in deferred cost of sales recognized as $ 292.8 million of deferred income on shipments to distributors. The deferred income on shipments to distributors that will ultimately be recognized in the Company's income statement will be lower than the amount reflected on the balance sheet due to price credits to be granted to the distributors when the product is sold to their customers. These price credits historically have resulted in the deferred income approximating the overall gross margins that the Company recognizes in the distribution channel of its business. The Company reduces product pricing through price protection based on market conditions, competitive considerations and other factors. Price protection is granted to distributors on the inventory they have on hand at the date the price protection is offered. When the Company reduces the price of its products, it allows the distributor to claim a credit against its outstanding accounts receivable balances based on the new price of the inventory it has on hand as of the date of the price reduction. There is no immediate revenue impact from the price protection, as it is reflected as a reduction of the deferred income on shipments to distributors' balance. Products returned by distributors and subsequently scrapped have historically been immaterial to the Company's consolidated results of operations. The Company routinely evaluates the risk of impairment of the deferred cost of sales component of the deferred income on shipments to distributors' account. Because of the historically immaterial amounts of inventory that have been scrapped, and historically rare instances where discounts given to a distributor result in a price less than the Company's cost, the Company believes the deferred costs have a low risk of material impairment. Shipping charges billed to customers are included in net sales, and the related shipping costs are included in cost of sales. The Company collects and remits certain sales-related taxes on a portion of its sales of inventory and reports such amounts under the net method in its consolidated statements of income. For licenses or other technology arrangements without an upgrade period, non-royalty revenue from the license is recognized upon delivery of the technology if the fee is fixed or determinable and collection of the fee is reasonably assured. Royalties are recognized when reported to the Company, which generally coincides with the receipt of payment. In certain limited circumstances, the Company enters into license and other arrangements for technologies that the Company is continuing to enhance and refine or under which it is obligated to provide unspecified enhancements. Under these arrangements, non-royalty revenue is recognized over the lesser of (1) the estimated period that the Company has historically enhanced and developed refinements to the specific technology, typically one to three years (the "upgrade period"), and (2) the remaining portion of the upgrade period after the date of delivery of all specified technology and documentation, provided that the fee is fixed or determinable and collection of the fee is reasonably assured. Royalties received during the upgrade period are recognized as revenue based on an amortization calculation of the elapsed portion of the upgrade period compared to the entire estimated upgrade period. Royalties received after the upgrade period has elapsed are recognized when reported to the Company, which generally coincides with the receipt of payment. Changes to Revenue Recognition Resulting From the Adoption of ASC 606 In May 2014, the Financial Accounting Standards Board (FASB) issued ASU 2014-09 -Revenue from Contracts with Customers (Topic 606) , which superseded nearly all existing revenue recognition guidance under generally accepted accounting principles in the United States of America (US GAAP). In August 2015, the FASB issued ASU 2015-14- Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delayed the effective date of the new standard by one year to December 15, 2017, for annual and interim reporting periods beginning after that date. In accordance with the delay, the new standard was effective for the Company beginning April 1, 2018. The standard's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The new standard allows for the amendment to be applied either retrospectively to each prior reporting period presented or retrospectively as a cumulative-effect adjustment as of the date of adoption. In March 2016, the FASB issued ASU 2016-08- Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net) , which clarifies the implementation guidance on principal versus agent considerations. In April 2016, the FASB issued ASU 2016-10- Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , which clarifies the implementation guidance on identifying performance obligations. In May 2016, the FASB issued ASU 2016-12- Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients , which addresses implementation issues that were raised by stakeholders and discussed by the Revenue Recognition Transition Resource Group. For a discussion of the financial statement impact related to the adoption of these standards, see “Recently Issued Accounting Pronouncements Not Yet Adopted”. Product Warranty The Company typically warrants its products against defects in materials and workmanship and non-conformance to specifications for 12 to 24 months. The majority of the Company's product warranty claims are settled through the return of the defective product and the shipment of replacement product. Warranty returns are included within the Company's allowance for returns, which is based on historical return rates. Actual future returns could differ from the allowance established. In addition, the Company accrues a liability for specific warranty costs expected to be settled other than through product return and replacement, if a loss is probable and can be reasonably estimated. Product warranty expenses were immaterial for the fiscal years ended March 31, 2018 , 2017 , and 2016 . Advertising Costs The Company expenses all advertising costs as incurred. Advertising costs were immaterial for the fiscal years ended March 31, 2018 , 2017 and 2016 . Research and Development Research and development costs are expensed as incurred. Assets purchased to support the Company's ongoing research and development activities are capitalized when related to products which have achieved technological feasibility or that have alternative future uses and are amortized over their estimated useful lives. Renewals or extensions of these assets are expensed as incurred. Research and development expenses include expenditures for labor, share-based payments, depreciation, masks, prototype wafers, and expenses for development of process technologies, new packages, and software to support new products and design environments. Restructuring Charges The Company recognizes a liability measured at fair value for restructuring costs when the liability is incurred. Restructuring charges are included within special charges and other, net in the consolidated statements of income and are primarily comprised of employee separation costs, asset impairments, contract exit costs and costs of facility consolidation and closure, including the related gains or losses associated with the sale of owned facilities. Employee separation costs includes one-time termination benefits that are recognized as a liability at estimated fair value, at the time of communication to employees, unless future service is required, in which case the costs are recognized ratably over the future service period. Ongoing termination benefits are recognized as a liability at estimated fair value when the amount of such benefits are probable and reasonably estimable. Contract exit costs includes contract termination fees and future contractual commitments for lease payments. A liability for contract exit costs is recognized in the period in which the Company terminates the contract or on the cease-use date for leased facilities. Foreign Currency Translation The Company's foreign subsidiaries are considered to be extensions of the U.S. company and any translation gains and losses related to these subsidiaries are included in other income (expense) in the consolidated statements of income. As the U.S. dollar is utilized as the functional currency, gains and losses resulting from foreign currency transactions (transactions denominated in a currency other than the subsidiaries' functional currency) are also included in income. For a portion of fiscal 2017, certain foreign subsidiaries acquired as part of the Company's acquisition activities had the local currency as the functional currency. Once these entities were integrated into the Company's legal structure and intercompany agreements were executed, the U.S. dollar became the functional currency for such entities. Income Taxes As part of the process of preparing its consolidated financial statements, the Company is required to estimate its income taxes in each of the jurisdictions in which it operates. This process involves estimating its actual current tax exposure together with assessing temporary differences resulting from differing treatment of items for tax and accounting purposes. These differences result in deferred tax assets and liabilities, which are included within the Company's consolidated balance sheets. The Company must then assess the likelihood that its deferred tax assets will be recovered from future taxable income within the relevant jurisdiction and to the extent the Company believes that recovery is not likely, it must establish a valuation allowance. The Company provided valuation allowances for certain of its deferred tax assets, including state net operating loss carryforwards and state tax credits, where it is more likely than not that some portion, or all of such assets, will not be realized. Due to the Tax Cuts and Jobs Act (the "Act"), the Company released its valuation allowance on foreign tax credits during the period ending March 31, 2018, which was provisional. The Company is still evaluating how the Act impacts its valuation allowance on state net operating loss carryforwards and state tax credits, and it may report an adjustment to the valuation allowances under Staff Accounting Bulletin ("SAB") 118 in subsequent quarters. Should the Company determine that it would not be able to realize all or part of its net deferred tax asset in the future, an adjustment to the deferred tax asset would be charged to income in the period such determination was made. Various taxing authorities in the U.S. and other countries in which the Company does business scrutinize the tax structures employed by businesses. Companies of a similar size and complexity as the Company are regularly audited by the taxing authorities in the jurisdictions in which they conduct significant operations. During the year ended March 31, 2018, various foreign jurisdictions finalized their audits. The close of these audits did not have an adverse impact on our financial statements. The Company is currently being audited by the tax authorities in the United States and various foreign jurisdictions. At this time, the Company does not know what the outcome of these audits will be. We record benefits for uncertain tax positions based on an assessment of whether it is more likely than not that the tax positions will be sustained based on their technical merits under currently enacted law. If this threshold is not met, no tax benefit of the uncertain tax position is recognized. If the threshold is met, we recognize the largest amount of the tax benefit that is more than 50% likely to be realized upon ultimate settlement. The accounting model as defined in Accounting Standards Codification ("ASC") 740 related to the valuation of uncertain tax positions requires the Company to presume that the tax position will be examined by the relevant taxing authority that has full knowledge of all relevant information and that each tax position will be evaluated without consideration of the possibility of offset or aggregation with other positions. The recognition requirement for the liability exists even if the Company believes the possibility of examination by a taxing authority or discovery of the related risk matters is remote or where it has a long history of the taxing authority not performing an exam or overlooking an issue. The Company will record an adjustment to a previously recorded position if new information or facts related to the position are identified in a subsequent period. All adjustments to the positions are recorded through the income statement. Generally, adjustments will be recorded in periods subsequent to the initial recognition if the taxing authority has completed an audit of the period or if the statute of limitation expires. Due to the inherent uncertainty in the estimation process and in consideration of the criteria of the accounting model, amounts recognized in the financial statements in periods subsequent to the initial recognition may significantly differ from the estimated exposure of the position under the accounting model. On December 22, 2017, the Act was enacted into law. The Act provides for numerous significant tax law changes and modifications including the reduction of the U.S. federal corporate income tax rate from 35.0% to 21.0%, the requirement for companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and the creation of new taxes on certain foreign-sourced earnings. As a fiscal year-end taxpayer, certain provisions of the Act began to impact the Company in the third quarter of fiscal 2018, while other provisions will impact the Company beginning in fiscal 2019. The corporate tax rate reduction is effective as of January 1, 2018. Since the Company has a fiscal year rather than a calendar year, it is subject to rules relating to transitional tax rates. As a result, the Company's fiscal 2018 federal statutory rate will be a blended rate of 31.5% . In addition to the impacts of tax reform on fiscal 2018, the Act establishes new tax laws that will be effective for fiscal 2019, including, but not limited to, (1) a new provision designed to tax low-taxed income of foreign subsidiaries, which allows for the possibility of using foreign tax credits ("FTCs") and a deduction of up to 50% to offset the income tax liability (subject to some limitations); (2) limitations on the deductibility of certain executive compensation; (3) limitations on the deductibility of interest expense; and (4) limitations on the use of FTCs to reduce the U.S. income tax liability. While each of these provisions is expected to have an impact on the Company's tax expense for fiscal 2019 and future periods, the Company expects the tax on low-taxed income of foreign subsidiaries to have the most significant, adverse impact on its results. Due to the complexity of the new tax on low-taxed income of foreign subsidiaries, the Company is continuing to evaluate this provision of the Act and the application of ASC 740. Based on recent FASB deliberations, it appears the Company will be allowed to make an accounting policy choice of either (1) treating taxes due on future U.S. inclusions in taxable income as a current-period expense when incurred or (2) factoring such amounts into its measurement of deferred taxes. The Company's selection of an accounting policy will depend, in part, on analyzing its facts to determine what the impact is expected to be under each method. Cash and Cash Equivalents All highly liquid investments, including marketable securities with an original maturity to the Company of three months or less when acquired are considered to be cash equivalents. Available-for-Sale Investments The Company classifies its investments in debt and marketable equity securities as available-for-sale based upon management's intent with regard to the investments and the nature of the underlying securities. The Company's available-for-sale investments consist of government agency bonds, municipal bonds, corporate bonds and marketable equity securities. The Company's investments are carried at fair value with unrealized gains and losses reported in stockholders' equity unless losses are considered to be other than temporary impairments in which case the losses are recognized through the statement of income. Premiums and discounts are amortized or accreted over the life of the related available-for-sale security. Dividend and interest income are recognized when earned. The cost of available-for-sale debt securities sold is calculated using the first-in, first-out (FIFO) basis at the individual security level for sales from multiple lots. For sales of marketable equity securities, the Company uses an average cost basis at the individual security level. The Company includes within short-term investments its income yielding available-for-sale securities that can be readily converted to cash and includes within long-term investments those income yielding available-for-sale securities with maturities of over one year that have unrealized losses attributable to them or those that cannot be readily liquidated. As discussed in Note 4, if the Company anticipates that investments will be liquidated in anticipation of a pending merger, any unrealized losses are recognized as other-than-temporary impairments. If the anticipated liquidity event is within 12 months, the securities are classified as short-term investments. In the normal course of business, the Company intends and has the ability to hold its long-term investments with temporary impairments until such time as these assets are no longer impaired. As of March 31, 2018, such recovery of unrealized losses is not expected to occur within the next year as the investments will be sold. Derivative Instruments Derivative instruments are required to be recorded at fair value as either assets or liabilities in the Company's consolidated balance sheet. The Company's accounting policies for derivative instruments depends on whether the instrument has been designated and qualifies as part of a hedging relationship and further, on the type of hedging relationship. The Company does not apply hedge accounting to foreign currency forward contracts. Gains and losses associated with currency rate changes on forward contracts are recorded currently in income. These gains and losses have been immaterial to the Company's financial statements. The Company is exposed to fluctuations in prices for energy that it consumes, particularly electricity and natural gas. The Company also enters into variable-priced contracts for some purchases of electricity and natural gas, on an index basis. The Company seeks, or may seek, to partially mitigate these exposures through fixed-price contracts. These contracts meet the characteristics of derivative instruments, but generally qualify for the “normal purchases or normal sales” exception under authoritative guidance and require no mark-to-market adjustment. Interest rate derivative instruments designated as fair value hedges are designed to manage the exposure to interest rate movements and to reduce borrowing costs by converting fixed-rate debt into floating-rate debt. Under these agreements, the Company agrees to exchange, at specified intervals, the difference between the fixed and floating interest amounts calculated by reference to an agreed-upon notional principal amount. For derivative instruments that are designated and qualify as fair value hedges, the gain or loss on the derivatives as well as the offsetting gain or loss on the hedged item attributable to the hedged risk are recognized in earnings. The Company evaluates hedge effectiveness at inception and on an ongoing basis. If a derivative is no longer expected to be highly effective, hedge accounting is discontinued. The Company terminated its interest rate derivative instruments in fiscal 2016. Allowance for Doubtful Accounts The Company maintains an allowance for doubtful accounts for probable losses on uncollectible accounts receivable resulting from the inability of its customers to make required payments, which is included in bad debt expense. The Company determines the adequacy of this allowance by routinely analyzing the composition of accounts receivable and evaluating customer creditworthiness, credit history, facts and circumstances specific to outstanding balances and current economic conditions. Inventories Inventories are valued at the lower of cost or net realizable value using the first-in, first-out method. The Company writes down its inventory for estimated obsolescence or unmarketable inventory in an amount equal to the difference between the cost of inventory and the estimated net realizable value based upon assumptions about future demand and market conditions. If actual market conditions are less favorable than those projected by the Company, additional inventory write-downs may be required. Inventory impairment charges establish a new cost basis for inventory and charges are not subsequently reversed to income even if circumstances later suggest that increased carrying amounts are recoverable. In estimating reserves for obsolescence, the Company primarily evaluates estimates of demand over a 12-month period and provides reserves for inventory on hand in excess of the estimated 12-month demand. Estimates for projected 12-month demand are generally based on the average shipments of the prior three-month period, which are then annualized to adjust for any potential seasonality in the Company's business. The estimated 12-month demand is compared to the Company's most recently developed sales forecast to further reconcile the 12-month demand estimate. Management reviews and adjusts the estimates as appropriate based on specific situations. For example, demand can be adjusted up for new products for which historic sales are not representative of future demand. Alternatively, demand can be adjusted down to the extent any existing products are being replaced or discontinued. In periods where the Company's production levels are substantially below normal operating capacity, unabsorbed overhead production costs associated with the reduced production levels of the Company's manufacturing facilities are charged directly to cost of sales. Property, Plant and Equipment Property, plant and equipment are stated at cost. Major renewals and improvements are capitalized, while maintenance and repairs are expensed when incurred. The Company's property and equipment accounting policies incorporate estimates, assumptions and judgments relative to the useful lives of its property and equipment. Depreciation is provided for assets placed in service on a straight-line basis over the estimated useful lives of the relative assets, which range from 10 to 40 years for buildings and building improvements and 3 to 7 years for machinery and equipment. The Company evaluates the carrying value of its property and equipment when events or changes in circumstances indicate that the carrying value of such assets may be impaired. Asset impairment evaluations are, by nature, highly subjective. Senior and Junior Subordinated Convertible Debt The Company separately accounts for the liability and equity components of its senior and junior subordinated convertible debt in a manner that reflects its nonconvertible debt (unsecured debt) borrowing rate when interest cost is recognized. This results in a bifurcation of a component of the debt, classification of that component in equity and the accretion of the resulting discount on the debt to be recognized as part of interest expense in its consolidated statements of income. Lastly, the Company includes the dilutive effect of the shares of its common stock issuable upon conversion of the outstanding senior and junior subordinated convertible debt in its diluted income per share calculation regardless of whether the market price triggers or other contingent conversion features have been met. The Company applies the treasury stock method as it has the intent and ability to settle the principal amounts of the senior and junior subordinated convertible debentures in cash. This method results in incremental dilutive shares when the average market value of the Company's common stock for a reporting period exceeds the conversion prices per share and adjust as dividends are recorded in the future. Upon a de-recognition event such as a settlement or conversion, the Company estimates the fair value of the liability component and compares that to the carrying amount in order to calculate the appropriate amount of gain or loss. The remaining amounts paid or issued (in the case of non cash consideration in the form of shares of common stock) are recognized as a reduction of additional paid-in-capital. The fair value of the liability c

Business Acquisitions

Business Acquisitions	12 Months Ended
	Mar. 31, 2018
Business Combinations [Abstract]
Business Acquisitions	Business Acquisitions Acquisition of Atmel On April 4, 2016, the Company acquired Atmel, a publicly traded company based in San Jose, California. The Company paid an aggregate of approximately $2.98 billion in cash and issued an aggregate of 10.1 million shares of its common stock to Atmel stockholders valued at $486.1 million based on the closing price of the Company's common stock on April 4, 2016 and incurred transaction and other fees of approximately $ 14.9 million . The total consideration transferred in the acquisition, including approximately $ 7.5 million of non-cash consideration for the exchange of certain share-based payment awards of Atmel for stock awards of the Company, was approximately $ 3.47 billion . In addition to the consideration transferred, the Company recognized in its consolidated financial statements $653.0 million in liabilities of Atmel consisting of debt, taxes payable and deferred, pension obligations, restructuring, and contingent and other liabilities. The Company financed the cash portion of the purchase price using approximately $ 2.04 billion of cash held by certain of its foreign subsidiaries and approximately $ 0.94 billion from additional borrowings under its existing credit agreement. As a result of the acquisition, Atmel became a wholly owned subsidiary of the Company. Atmel is a worldwide leader in the design and manufacture of microcontrollers, capacitive touch solutions, advanced logic, mixed-signal, nonvolatile memory and radio frequency components. The Company's primary reason for this acquisition was to expand the Company's range of solutions, products and capabilities by extending its served available market. The acquisition was accounted for under the acquisition method of accounting, with the Company identified as the acquirer, and the operating results of Atmel have been included in the Company's consolidated financial statements as of the closing date of the acquisition. Under the acquisition method of accounting, the aggregate amount of consideration paid by the Company was allocated to Atmel's net tangible assets and intangible assets based on their estimated fair values as of April 4, 2016. The excess of the purchase price over the value of the net tangible assets and intangible assets was recorded to goodwill. The factors contributing to the recognition of goodwill were based upon the Company's conclusion that there are strategic and synergistic benefits that are expected to be realized from the acquisition. The goodwill has been allocated to the Company's semiconductor products reporting segment. None of the goodwill related to the Atmel acquisition is deductible for tax purposes. The Company retained independent third-party appraisers to assist management in its valuation. The table below represents the allocation of the final purchase price to the net assets acquired based on their estimated fair values, as well as the associated estimated useful lives of the acquired intangible assets (amounts in millions). Assets acquired Cash and cash equivalents $ 230.2 Accounts receivable 141.4 Inventories 335.1 Prepaid expenses and other current assets 28.4 Assets held for sale 32.0 Property, plant and equipment 129.9 Goodwill 1,286.4 Purchased intangible assets 1,888.4 Long-term deferred tax assets 46.7 Other assets 7.5 Total assets acquired 4,126.0 Liabilities assumed Accounts payable (55.7 ) Other current liabilities (121.0 ) Long-term line of credit (192.0 ) Deferred tax liabilities (27.5 ) Long-term income tax payable (115.1 ) Other long-term liabilities (141.7 ) Total liabilities assumed (653.0 ) Purchase price allocated $ 3,473.0 Purchased Intangible Assets Weighted Average Useful Life April 4, 2016 (in years) (in millions) Core and developed technology 11 $ 1,075.0 In-process research and development — 140.7 Customer-related 6 630.6 Backlog 1 40.3 Other 5 1.8 Total purchased intangible assets $ 1,888.4 Purchased intangible assets include core and developed technology, in-process research and development, customer-related intangibles, acquisition-date backlog and other intangible assets. The estimated fair values of the core and developed technology and in-process research and development were determined based on the present value of the expected cash flows to be generated by the respective existing technology or future technology. The core and developed technology intangible assets are being amortized in a manner based on the expected cash flows used in the initial determination of fair value. In-process research and development is capitalized until such time as the related projects are completed or abandoned at which time the capitalized amounts will begin to be amortized or written off. Customer-related intangible assets consist of Atmel's contractual relationships and customer loyalty related to its distributor and end-customer relationships, and the fair values of the customer-related intangibles were determined based on Atmel's projected revenues. An analysis of expected attrition and revenue growth for existing customers was prepared from Atmel's historical customer information. Customer relationships are being amortized in a manner based on the estimated cash flows associated with the existing customers and anticipated retention rates. Backlog relates to the value of orders not yet shipped by Atmel at the acquisition date, and the fair values were based on the estimated profit associated with those orders. Backlog related assets had a one year useful life and were being amortized on a straight-line basis over that period. The total weighted average amortization period of intangible assets acquired as a result of the Atmel transaction is 9 years. Amortization expense associated with acquired intangible assets is not deductible for tax purposes. Thus, approximately $ 178.1 million was established as a net deferred tax liability for the future amortization of the intangible assets. Acquisition of Micrel On August 3, 2015, the Company acquired Micrel, Incorporated (Micrel), a publicly traded company based in San Jose, California. The Company paid an aggregate of approximately $ 430.0 million in cash and issued an aggregate of 8.6 million shares of its common stock to Micrel shareholders. The number of shares issued in the transaction was subsequently repurchased by the Company in the open market during the fiscal year ended March 31, 2016. The total consideration transferred in the acquisition, including approximately $ 4.1 million of non cash consideration for the exchange of certain share-based payment awards of Micrel for stock awards of the Company, and approximately $ 13.1 million of cash consideration for the payout of vested employee stock awards, was approximately $ 816.2 million . The Company financed the cash portion of the purchase price using amounts borrowed under its credit agreement. As a result of the acquisition, Micrel became a wholly owned subsidiary of the Company. Micrel's business is to design, develop, manufacture and market a range of high-performance analog, power and mixed-signal integrated circuits. Micrel's products address a wide range of end markets including industrial, automotive and communications. Micrel also manufactures custom analog and mixed-signal circuits and provides wafer foundry services for customers which produce electronic systems utilizing semiconductor manufacturing processes as well as micro-electrical mechanical system technologies. The Company's primary reason for this acquisition was to expand the Company's range of solutions, products and capabilities by extending its served available market. The acquisition was accounted for under the acquisition method of accounting, with the Company identified as the acquirer, and the operating results of Micrel have been included in the Company's consolidated financial statements as of the closing date of the acquisition. Under the acquisition method of accounting, the aggregate amount of consideration paid by the Company was allocated to Micrel's net tangible assets and intangible assets based on their estimated fair values as of August 3, 2015. The excess of the purchase price over the value of the net tangible assets and intangible assets was recorded to goodwill. The factors contributing to the recognition of goodwill were based upon the Company's conclusion that there are strategic and synergistic benefits that are expected to be realized from the acquisition. The goodwill has been allocated to the Company's semiconductor products reporting segment. None of the goodwill related to the Micrel acquisition is deductible for tax purposes. The Company retained an independent third-party appraiser to assist management in its valuation. The table below represents the allocation of the purchase price to the net assets acquired based on their estimated fair values as of August 3, 2015, as well as the associated estimated useful lives of the acquired intangible assets at that date. The purchase price allocation was finalized as of June 30, 2016 (amounts in millions): Assets acquired Cash and cash equivalents $ 99.1 Accounts receivable, net 14.0 Inventories 73.5 Prepaid expenses and other current assets 10.7 Property, plant and equipment, net 38.5 Goodwill 441.0 Purchased intangible assets 273.5 Other assets 4.3 Total assets acquired 954.6 Liabilities assumed Accounts payable (11.1 ) Other current liabilities (31.6 ) Deferred tax liabilities (88.0 ) Long-term income tax payable (7.6 ) Other long-term liabilities (0.1 ) Total liabilities assumed (138.4 ) Purchase price allocated $ 816.2 Purchased Intangible Assets Weighted Average Useful Life August 3, 2015 (in years) (in millions) Core and developed technology 10 $ 175.8 In-process research and development — 21.0 Customer-related 5 71.1 Backlog 1 5.6 Total purchased intangible assets $ 273.5 Purchased intangible assets include core and developed technology, in-process research and development, customer-related intangibles and acquisition-date backlog. The estimated fair values of the core and developed technology and in-process research and development were determined based on the present value of the expected cash flows to be generated by the respective existing technology or future technology. The core and developed technology intangible assets are being amortized commensurate with the expected cash flows used in the initial determination of fair value. In-process research and development is capitalized until such time as the related projects are completed or abandoned at which time the capitalized amounts will begin to be amortized or written off. Customer-related intangible assets consist of Micrel's contractual relationships and customer loyalty related to its distributor and end-customer relationships, and the fair values of the customer-related intangibles were determined based on Micrel's projected revenues. An analysis of expected attrition and revenue growth for existing customers was prepared from Micrel's historical customer information. Customer relationships are being amortized in a manner consistent with the estimated cash flows associated with the existing customers and anticipated retention rates. Backlog relates to the value of orders not yet shipped by Micrel at the acquisition date, and the preliminary fair values were based on the estimated profit associated with those orders. Backlog related assets are being recognized commensurate with recognition of the revenue for the orders on which the backlog intangible assets were determined. Amortization expense associated with acquired intangible assets is not deductible for tax purposes. Thus, approximately $ 99.7 million was established as a net deferred tax liability for the future amortization of the intangible assets offset by $11.4 million of net deferred tax assets.

Special Charges and Other, Net

Special Charges and Other, Net	12 Months Ended
	Mar. 31, 2018
Other Nonrecurring (Income) Expense [Abstract]
Special Charges and Other, Net	Special Charges and Other, Net The following table summarizes activity included in the "special charges and other, net" caption on the Company's consolidated statements of income (amounts in millions): For The Years Ended March 31, 2018 2017 2016 Restructuring Employee separation costs $ 1.2 $ 39.1 $ 9.6 Gain on sale of assets (4.4 ) — — Impairment charges — 12.6 — Contract exit costs 0.7 44.1 0.7 Other — 2.8 0.9 Legal settlement costs — — 4.3 Insurance settlement — — (11.5 ) Non-restructuring contract exit costs and other $ 20.0 $ — $ — Total $ 17.5 $ 98.6 $ 4.0 The Company continuously evaluates its existing operations in an attempt to identify and realize cost savings opportunities and operational efficiencies. This same approach is applied to businesses that are acquired by the Company and often the operating models of acquired companies are not as efficient as the Company's operating model which enables the Company to realize significant savings and efficiencies. As a result, following an acquisition, the Company will from time to time incur restructuring expenses; however, the Company is often not able to estimate the timing or amount of such costs in advance of the period in which they occur. The primary reason for this is that the Company regularly reviews and evaluates each position, contract and expense against the Company's strategic objectives, long-term operating targets and other operational priorities. Decisions related to restructuring activities are made on a "rolling basis" during the course of the integration of an acquisition whereby department managers, executives and other leaders work together to evaluate each of these expenses and make recommendations. As a result of this approach, at the time of an acquisition and at March 31, 2018, the Company is not able to estimate the total or future amount of expected employee separation or exit costs that it will incur in connection with its restructuring activities. During fiscal 2018, the Company incurred expenses including non-restructuring contract exit costs of $19.5 million for fees associated with transitioning from the public utility provider in Oregon to a lower cost direct access provider. The fee is paid monthly and will depend on the amount of actual energy consumed by the Company's wafer fabrication facility in Oregon over the next five years. In connection with the transition to a direct access provider, the Company signed a ten -year supply agreement to purchase monthly amounts of energy that are less than the current average usage and priced on a per mega watt hour published index rate in effect at those future dates. Also during fiscal 2018, the Company incurred $1.2 million of employee separation costs in connection with the acquisition of Atmel. The Company may continue to incur additional costs in the future as additional synergies or operational efficiencies are identified. The Company is not able to estimate the amount of such future expenses, if any, at this time. The Company's restructuring expenses during fiscal 2017 were related to the Company's most recent business combinations, including the acquisitions of Atmel and Micrel, and resulted from workforce, property and other operating expense rationalizations as well as combining product roadmaps and manufacturing operations. These expenses were for employee separation costs, contract exit costs, other operating expenses and intangible asset impairment losses. At March 31, 2017, these activities were substantially complete. All of the Company's restructuring activities occurred in its semiconductor products segment. The Company incurred $52.2 million in costs since the start of fiscal 2015 in connection with employee separation activities, of which $1.2 million , $39.1 million and $9.6 million was incurred during the fiscal years ended March 31, 2018 , 2017 and 2016 , respectively. These employee separation activities are now substantially complete and any future amounts are not expected to be material. The Company has incurred $45.3 million in costs in connection with contract exit activities since the start of fiscal 2015 which includes $0.7 million , $44.1 million and $0.7 million incurred for the years ended March 31, 2018 , 2017 and 2016 , respectively. These acquisition-related contract exit activities are substantially complete and any future amounts are not expected to be material. In the three months ended June 30, 2017, the Company completed the sale of an asset it acquired as part of its acquisition of Micrel for proceeds of $10.0 million and the gain of $4.4 million is included in the gain on sale of assets in the above table. As of March 31, 2017, these assets consisting of property, plant and equipment were presented as held for sale in the Company's consolidated financial statements. The impairment charges in fiscal 2017 were recognized as a result of changes in the combined product roadmaps after the acquisition of Atmel that affected the use and life of these assets. The following is a roll forward of accrued restructuring charges for fiscal 2018 and fiscal 2017 (amounts in millions): Employee Separation Costs Exit Costs Total Balance at March 31, 2016 - Restructuring Accrual $ 0.1 $ — $ 0.1 Additions due to Atmel acquisition 6.3 — 6.3 Charges 39.1 44.1 83.2 Payments (38.9 ) (7.0 ) (45.9 ) Non-cash - Other (0.5 ) (2.3 ) (2.8 ) Changes in foreign exchange rates (0.7 ) — (0.7 ) Balance at March 31, 2017 - Restructuring Accrual 5.4 34.8 40.2 Charges 1.2 0.7 1.9 Payments (5.9 ) (9.2 ) (15.1 ) Non-cash - Other (0.2 ) 1.0 0.8 Changes in foreign exchange rates 0.3 — 0.3 Balance at March 31, 2018 - Restructuring Accrual $ 0.8 $ 27.3 $ 28.1 Current $ 11.9 Non-current 16.2 Total $ 28.1 The restructuring liability of $28.1 million is included in accrued liabilities and other long-term liabilities, on the Company's consolidated balance sheets as of March 31, 2018 .

Investments

Investments	12 Months Ended
	Mar. 31, 2018
Investments [Abstract]
Investments	Investments The Company's investments are intended to establish a high-quality portfolio that preserves principal, meets liquidity needs, avoids inappropriate concentrations, and delivers an appropriate yield in relationship to the Company's investment guidelines and market conditions. 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 At March 31, 2018 , all of the Company's available-for-sale securities are presented on the consolidated balance sheets as short-term investments of $ 1,295.3 million . All available-for-sale debt securities are classified as short-term on the consolidated balance sheet because the Company intends to sell them in the first quarter of fiscal 2019 to fund the closing of its pending acquisition of Microsemi. At March 31, 2017 , the Company's available-for-sale securities are presented on the consolidated balance sheets as short-term investments of $ 394.1 million and long-term investments of $ 107.5 million . There were no sales of available-for-sale investments during the year ended March 31, 2018 . The Company sold available-for-sale investments for proceeds of $ 470.2 million and $ 1,501.5 million during the years ended March 31, 2017 and 2016 , respectively. The Company had no material net realized gains from sales of available-for-sale marketable equity and debt securities during the years ended March 31, 2018 and March 31, 2017 compared to gains of $ 13.7 million during the year ended March 31, 2016 . During the three months ended March 31, 2018, the Company recognized an impairment of $ 15.5 million on available-for-sale investments based on its evaluation of available evidence and the Company's intent to sell these investments during the first quarter of fiscal 2019 to fund its pending acquisition of Microsemi. The Company determines the cost of available-for-sale debt securities sold on a first-in first-out (FIFO) basis at the individual security level for sales from multiple lots. For sales of marketable equity securities, the Company uses an average cost basis at the individual security level. Gains and losses recognized in earnings are credited or charged to other income (expense) on the consolidated statements of income. As of March 31, 2018, the Company did not have any investments in an unrealized position for which an other-than-temporary impairment had not been recognized. 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 ) 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

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Mar. 31, 2018
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Fair Value Measurements	Fair Value Measurements Accounting rules for fair value clarify that fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the Company utilizes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows: Level 1- Observable inputs such as quoted prices in active markets; Level 2- Inputs, other than the quoted prices in active markets, that are observable either directly or indirectly; and Level 3- Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions. Marketable Debt Instruments Marketable debt instruments include instruments such as corporate bonds and debt, government agency bonds, bank deposits, municipal bonds, and money market mutual funds. When the Company uses observable market prices for identical securities that are traded in less active markets, the Company classifies its marketable debt instruments as Level 2. When observable market prices for identical securities are not available, the Company prices its marketable debt instruments using non-binding market consensus prices that are corroborated with observable market data; quoted market prices for similar instruments; or pricing models, such as a discounted cash flow model, with all significant inputs derived from or corroborated with observable market data. Non-binding market consensus prices are based on the proprietary valuation models of pricing providers or brokers. These valuation models incorporate a number of inputs, including non-binding and binding broker quotes; observable market prices for identical or similar securities; and the internal assumptions of pricing providers or brokers that use observable market inputs and, to a lesser degree, unobservable market inputs. The Company corroborates non-binding market consensus prices with observable market data using statistical models when observable market data exists. The discounted cash flow model uses observable market inputs, such as LIBOR-based yield curves, currency spot and forward rates, and credit ratings. Assets Measured at Fair Value on a Recurring Basis Assets measured at fair value on a recurring basis at March 31, 2018 are as follows (amounts in millions): Quoted Prices Significant Other Observable Inputs (Level 2) Total Balance Assets Cash and cash equivalents: Money market mutual funds $ 121.0 $ — $ 121.0 Deposit accounts — 641.6 641.6 Commercial Paper — 118.7 118.7 Government agency bonds — 20.0 20.0 Short-term investments: Marketable equity securities 2.8 — 2.8 Corporate bonds and debt — 542.9 542.9 Time deposits — 11.5 11.5 Government agency bonds — 723.2 723.2 Municipal bonds - taxable — 14.9 14.9 Total assets measured at fair value $ 123.8 $ 2,072.8 $ 2,196.6 Assets measured at fair value on a recurring basis at March 31, 2017 are as follows (amounts in millions): Quoted Prices Significant Other Observable Inputs (Level 2) Total Balance Assets Cash and cash equivalents: Money market mutual funds $ 343.8 $ — $ 343.8 Deposit accounts — 564.9 564.9 Short-term investments: Marketable equity securities 1.6 — 1.6 Corporate bonds and debt — 165.2 165.2 Government agency bonds — 162.0 162.0 Municipal bonds - tax-exempt — 55.3 55.3 Municipal bonds - taxable — 10.0 10.0 Long-term Investments: Corporate bonds and debt — 42.6 42.6 Government agency bonds — 64.9 64.9 Total assets measured at fair value $ 345.4 $ 1,064.9 $ 1,410.3 There were no transfers between Level 1 or Level 2 during fiscal 2018 or fiscal 2017 . There were no assets measured on a recurring basis during fiscal 2018 or fiscal 2017 using significant unobservable inputs (Level 3). Assets and Liabilities Measured and Recorded at Fair Value on a Non-Recurring Basis The Company's non-marketable equity, cost method investments, certain acquired liabilities and non-financial assets, such as intangible assets, assets held for sale and property, plant and equipment, are recorded at fair value on a non-recurring basis. These assets are subject to fair value adjustments in certain circumstances, for example, when there is evidence of impairment. The Company's non-marketable and cost method investments are monitored on a quarterly basis for impairment charges. The fair values of these investments have been determined as Level 3 fair value measurements because the valuations use unobservable inputs that require management's judgment due to the absence of quoted market prices. There were no impairment charges recognized on these investments during the years ended March 31, 2018 , 2017 and 2016 . These investments are included in other assets on the consolidated balance sheets. The fair value measurements related to the Company's non-financial assets, such as intangible assets, assets held for sale and property, plant and equipment are based on available market prices at the measurement date based on transactions of similar assets and third-party independent appraisals, less costs to sell where appropriate. The Company classifies these measurements as Level 2.

Fair Value of Financial Instrum

Fair Value of Financial Instruments	12 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amount of cash equivalents approximates fair value because their maturity is less than three months. Management believes the carrying amount of the equity and cost-method investments materially approximated fair value at March 31, 2018 based upon unobservable inputs. The fair values of these investments have been determined as Level 3 fair value measurements. The carrying amount of accounts receivable, accounts payable and accrued liabilities approximates fair value due to the short-term maturity of the amounts and are considered Level 2 in the fair value hierarchy. Fair Value of Subordinated Convertible Debt The Company measures the fair value of its senior and junior subordinated convertible debt for disclosure purposes. These fair values are based on observable market prices for this debt, which is traded in less active markets and are therefore classified as a Level 2 fair value measurement. The following table shows the carrying amounts and fair values of the Company’s senior and junior subordinated convertible debt as of March 31, 2018 and 2017 (amounts in millions). As of March 31, 2018 and March 31, 2017 , the carrying amounts of the Company's senior and junior subordinated convertible debt have been reduced by debt issuance costs of $34.2 million and $38.3 million , respectively. March 31, 2018 2017 Carrying Amount Fair Value Carrying Amount Fair Value 2017 Senior Debt $ 1,437.6 $ 2,459.2 $ 1,384.9 $ 2,106.2 2015 Senior Debt $ 1,309.9 $ 3,079.1 $ 1,261.8 $ 2,481.7 2017 Junior Debt $ 326.7 $ 876.9 $ 262.3 $ 586.6 2007 Junior Debt $ — $ — $ 50.0 $ 445.1

Other Financial Statement Detai

Other Financial Statement Details	12 Months Ended
	Mar. 31, 2018
Accounts Receivable, Net [Abstract]
Other Financial Statement Details	Other Financial Statement Details Accounts Receivable Accounts receivable consists of the following (amounts in millions): March 31, 2018 2017 Trade accounts receivable $ 557.8 $ 473.3 Other 8.1 7.2 Total accounts receivable, gross 565.9 480.5 Less allowance for doubtful accounts 2.2 2.1 Total accounts receivable, net $ 563.7 $ 478.4 Inventories The components of inventories consist of the following (amounts in millions): March 31, 2018 2017 Raw materials $ 26.0 $ 14.4 Work in process 311.8 268.3 Finished goods 138.4 134.5 Total inventories $ 476.2 $ 417.2 Inventories are valued at the lower of cost and net realizable value using the first-in, first-out method. Inventory impairment charges establish a new cost basis for inventory and charges are not subsequently reversed to income even if circumstances later suggest that increased carrying amounts are recoverable. Property, Plant and Equipment Property, plant and equipment consists of the following (amounts in millions): March 31, 2018 2017 Land $ 73.4 $ 73.4 Building and building improvements 508.5 499.7 Machinery and equipment 1,943.9 1,774.9 Projects in process 118.3 104.3 Total property, plant and equipment, gross 2,644.1 2,452.3 Less accumulated depreciation and amortization 1,876.2 1,769.0 Total property, plant and equipment, net $ 767.9 $ 683.3 Depreciation expense attributed to property, plant and equipment was $ 123.7 million , $ 122.9 million and $ 103.9 million for the fiscal years ending March 31, 2018 , 2017 and 2016 , respectively. During the quarter ended March 31, 2018, the Company entered into an agreement to purchase one of the buildings it leased in San Jose, California for a purchase price of $40.8 million with a closing date of April 4, 2018. During the quarter ended December 31, 2016, the Company began to actively market a 6-inch wafer fabrication facility it acquired as part of its acquisition of Micrel in August 2015. In the three months ended June 30, 2017, the Company completed the sale of these assets for proceeds of $10.0 million . As of March 31, 2017, these assets consisting of property, plant and equipment were presented as held for sale in the Company's consolidated financial statements.

Discontinued Operations

Discontinued Operations	12 Months Ended
	Mar. 31, 2018
Discontinued Operations and Disposal Groups [Abstract]
Discontinued Operations	Discontinued Operations Discontinued operations include the mobile touch operations that the Company acquired as part of its acquisition of Atmel. The mobile touch assets had been marketed for sale since the Company's acquisition of Atmel on April 4, 2016 based on management's decision that it was not a strategic fit for the Company's product portfolio. On November 10, 2016, the Company completed the sale of the mobile touch assets to Solomon Systech (Limited) International, a Hong Kong based semiconductor company. The transaction included the sale of certain semiconductor products, equipment, customer list, backlog, patents, and a license to certain other intellectual property and patents related to the Company's mobile touch product line. The Company also agreed to provide certain transition services to Solomon Systech, which were substantially complete as of March 31, 2017. For financial statement purposes, the results of operations for this discontinued business have been segregated from those of the continuing operations and are presented in the Company's consolidated financial statements as discontinued operations. As the Company completed the sale of the mobile touch assets on November 10, 2016, there are no discontinued operations in the year ended March 31, 2018 . The results of discontinued operations for the year ended March 31, 2017 are as follows (amounts in millions): March 31, 2017 Net sales $ 18.3 Cost of sales 15.8 Operating expenses 10.7 Gain on Sale 0.6 Income tax benefit (1.6 ) Net loss from discontinued operations $ (6.0 )

Intangible Assets and Goodwill

Intangible Assets and Goodwill	12 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets and Goodwill	Intangible Assets and Goodwill Intangible assets consist of the following (amounts in millions): March 31, 2018 Gross Amount Accumulated Amortization Net Amount Core and developed technology $ 1,952.3 $ (644.4 ) $ 1,307.9 Customer-related 716.9 (375.9 ) 341.0 In-process research and development 12.1 — 12.1 Distribution rights 0.3 (0.1 ) 0.2 Other 1.5 (0.7 ) 0.8 Total $ 2,683.1 $ (1,021.1 ) $ 1,662.0 March 31, 2017 Gross Amount Accumulated Amortization Net Amount Core and developed technology $ 1,932.3 $ (419.5 ) $ 1,512.8 Customer-related 716.9 (123.6 ) 593.3 Trademarks and trade names 11.7 (9.6 ) 2.1 In-process research and development 38.5 — 38.5 Distribution rights 5.6 (5.3 ) 0.3 Other 1.5 (0.4 ) 1.1 Total $ 2,706.5 $ (558.4 ) $ 2,148.1 The Company amortizes intangible assets over their expected useful lives, which range between 1 and 15 years. During the year ended March 31, 2018 , the Company acquired $ 7.1 million of core and developed technology which has a weighted average amortization period of 9 years. In fiscal 2018 , $ 26.2 million of in-process research and development intangible assets reached technological feasibility and was reclassified as core and developed technology and began being amortized over the respective estimated useful lives. The following is an expected amortization schedule for the intangible assets for fiscal 2019 through fiscal 2023 , absent any future acquisitions or impairment charges (amounts in millions): Fiscal Year Ending March 31, Projected Amortization Expense 2019 $362.8 2020 314.6 2021 259.3 2022 192.5 2023 143.7 Amortization expense attributed to intangible assets was $ 492.2 million , $ 346.3 million and $ 179.3 million for fiscal 2018 , 2017 and 2016 , respectively. In fiscal 2018 , $ 6.1 million was charged to cost of sales and $ 486.1 million was charged to operating expenses. In fiscal 2017 , $ 4.0 million was charged to cost of sales and $ 342.3 million was charged to operating expenses. In fiscal 2016 , $ 3.6 million was charged to cost of sales and $ 175.7 million was charged to operating expenses. The Company recognized impairment charges of $ 0.5 million and $ 0.6 million in fiscal 2018 and fiscal 2016 , respectively. During fiscal 2017, the Company recognized $ 11.9 million of intangible asset impairment changes, primarily as a result of the acquisition of Atmel. The impairment losses were recognized as a result of changes in the combined product roadmaps after the acquisition of Atmel that affected the use and life of these assets. Goodwill activity for fiscal 2018 and fiscal 2017 was as follows (amounts in millions): Semiconductor Products Reporting Unit Technology Licensing Reporting Unit Balance at March 31, 2016 $ 993.5 $ 19.2 Additions due to the acquisition of Atmel 1,286.3 — Balance at March 31, 2017 2,279.8 19.2 Balance at March 31, 2018 $ 2,279.8 $ 19.2 At March 31, 2018 , the Company applied a qualitative goodwill impairment test to its two reporting units, concluding it was not more likely than not that goodwill was impaired. Through March 31, 2018 , the Company has never recorded an impairment charge against its goodwill balance.

Income Taxes

Income Taxes	12 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The income tax provision consists of the following (amounts in millions): Year Ended March 31, 2018 2017 2016 Pretax Income: U.S. $ (127.3 ) $ (279.3 ) $ (75.5 ) Foreign 864.6 369.1 356.8 $ 737.3 $ 89.8 $ 281.3 Current expense (benefit): U.S. Federal $ 369.4 $ 21.3 $ (4.0 ) State 0.5 1.0 (0.2 ) Foreign 60.8 23.8 22.0 Total current $ 430.7 $ 46.1 $ 17.8 Deferred expense (benefit): U.S. Federal $ 82.5 $ (114.7 ) $ (42.2 ) State 0.1 (5.4 ) (2.0 ) Foreign (31.4 ) (6.8 ) (16.2 ) Total deferred 51.2 (126.9 ) (60.4 ) Total Income tax provision (benefit) $ 481.9 $ (80.8 ) $ (42.6 ) On December 22, 2017, the Tax Cuts and Jobs Act (the "Act") was enacted into law. The Act provides for numerous significant tax law changes and modifications including the reduction of the U.S. federal corporate income tax rate from 35.0% to 21.0%, the requirement for companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and the creation of new taxes on certain foreign-sourced earnings. As a fiscal year-end taxpayer, certain provisions of the Act began to impact the Company in the third quarter of fiscal 2018, while other provisions will impact the Company beginning in fiscal 2019. The corporate tax rate reduction is effective as of January 1, 2018. Since the Company has a fiscal year rather than a calendar year, it is subject to rules relating to transitional tax rates. As a result, the Company’s fiscal 2018 federal statutory rate will be a blended rate of 31.5% . Accounting Standards Codification ("ASC") 740, Income Taxes, requires companies to recognize the effect of the tax law changes in the period of enactment. However, the SEC staff issued Staff Accounting Bulletin ("SAB") 118 which allows companies to record provisional amounts during a measurement period that is similar to the measurement period used when accounting for business combinations. The Company recorded a reasonable estimate when possible and with the understanding that the provisional amount is subject to further adjustments under SAB 118. In addition, for significant items for which the Company could not make a reasonable estimate, no provisional amounts were recorded. Amounts will be recorded during the measurement period allowed under SAB 118 when a reasonable estimate can be made, or when the effect of the Act is known. As of March 31, 2018, the Company made a reasonable estimate of the effects on the one-time transition tax, its existing deferred tax balances and the release of its valuation allowances on foreign tax credits due to the Act, and the Company recognized a provisional amount of income tax expense of $471.6 million , which decreased diluted net income per common share by $1.89 for the fiscal year ended March 31, 2018, and which was included as a component of income tax provision from continuing operations. The Company will continue to refine what the provisional balances and adjustments may be made under SAB 118 during the measurement period as a result of future changes in interpretation, information available, assumptions made by the Company and/or issuance of additional guidance and these adjustments could be material. The one-time transition tax is based on the Company's total post-1986 earnings and profits ("E&P") of its foreign subsidiaries. Substantially all of the Company's E&P were permanently reinvested outside the U.S prior to the Act. The Company recorded provisional U.S. amounts for its one-time transition tax liabilities, resulting in an increase in income tax provision of $653.7 million . In addition, the Company released the deferred tax liabilities related to non-permanently reinvested E&P, resulting in a decrease in income tax provision of $9.0 million . The net increase to tax provision is $644.7 million . The one-time transition tax may be elected to be paid over a period of eight years. The Company intends to make this election. The Company has not yet completed its calculation of the total post-1986 E&P for its foreign subsidiaries. The one-time transition tax is based in part on the amount of those earnings held in cash and other specified assets either as of the end of fiscal 2018 or the average of the year-end balances for fiscal 2016 and fiscal 2017. The Company's calculation of this amount will change with further analysis and guidance from the U.S. federal and state tax authorities about the application of these new rules. The Company will continue to evaluate the impact of the tax law change as it relates to the accounting for the outside basis difference of its foreign entities. As a result of the reduction of the corporate income tax rate to 21.0%, U.S. GAAP requires companies to remeasure their deferred tax assets and liabilities as of the date of enactment, with resulting tax effects accounted for in the reporting period of enactment. The Company remeasured deferred tax assets and liabilities based on the rates at which they are expected to be utilized in the future. The provisional amount recorded for the remeasurement and resulting income tax benefit of the Company's deferred tax balance was $136.7 million . Due to the Act, the Company released its valuation allowance on foreign tax credits during the year ended March 31, 2018. The provisional amount recorded for the valuation allowance release was an income tax benefit of $36.4 million . The Company is still evaluating how the Act impacts the valuation allowance on state net operating loss carryforwards and state tax credits, and the Company may report an adjustment to the valuation allowances in accordance with SAB 118 in subsequent quarters. The provision for income taxes differs from the amount computed by applying the statutory federal tax rate to income before income taxes. The sources and tax effects of the differences in the total income tax provision are as follows (amounts in millions): Year Ended March 31, 2018 2017 2016 Computed expected income tax provision $ 232.6 $ 31.4 $ 98.4 Foreign income taxed at lower than the federal rate (208.8 ) (105.0 ) (120.1 ) Impact of the Act - one-time transition tax, net of foreign tax credits 653.7 — — Impact of the Act - deferred tax effects, net of valuation allowance (136.7 ) — — Increases related to current and prior year tax positions 32.0 53.7 14.5 Decreases related to prior year tax positions (1) (11.3 ) (36.3 ) (12.1 ) Share-based compensation (27.2 ) (25.0 ) — Research and development tax credits - current year (17.0 ) (12.8 ) (13.5 ) Research and development tax credits - prior years — — (2.5 ) Intercompany prepaid tax asset amortization 7.4 7.9 (15.5 ) Withholding taxes 1.4 5.6 6.0 Foreign exchange (20.5 ) (1.7 ) 5.6 Other (3.2 ) (0.4 ) (0.9 ) Change in valuation allowance (20.5 ) 1.8 (2.5 ) Total income tax provision (benefit) $ 481.9 $ (80.8 ) $ (42.6 ) (1) The release of prior year tax positions during fiscal 2018 increased the basic and diluted net income per common share by $0.05 . The release of prior year tax positions during fiscal 2017 increased the basic and diluted net income per common share by $0.17 and $0.15 , respectively. The release of prior year tax positions during fiscal 2016 increased the basic and diluted net income per common share by $0.06 . The foreign tax rate differential benefit primarily relates to the Company's operations in Thailand, Cayman and Ireland. The Company's Thailand manufacturing operations are currently subject to numerous tax holidays granted to the Company based on its investment in property, plant and equipment in Thailand. The Company's tax holiday periods in Thailand expire between 2022 and 2025, however, the Company actively seeks to obtain new tax holidays. The Company does not expect the future expiration of any of its tax holiday periods in Thailand to have a material impact on its effective tax rate. The aggregate dollar benefits derived from these tax holidays approximated $ 6.2 million , $ 13.2 million and $ 9.4 million in fiscal 2018 , 2017 and 2016 , respectively. The impact of the tax holidays during fiscal 2018 increased the basic and diluted net income per common share by $0.03 and $0.02 , respectively. The impact of the tax holidays during fiscal 2017 increased the basic and diluted net income per common share by $0.06 . The impact of the tax holidays during fiscal 2016 increased the basic and diluted net income per common share by $0.05 and $0.04 , respectively. The tax effects of temporary differences that give rise to significant portions of the Company's deferred tax assets and deferred tax liabilities are as follows (amounts in millions): March 31, 2018 2017 Deferred tax assets: Deferred income on shipments to distributors $ 39.1 $ 55.7 Inventory valuation 10.7 14.6 Net operating loss carryforward 101.1 91.6 Capital loss carryforward 10.6 12.9 Share-based compensation 31.4 42.5 Income tax credits 178.4 243.0 Property, plant and equipment 25.7 59.7 Accrued expenses and other 91.2 110.4 Gross deferred tax assets 488.2 630.4 Valuation allowances (204.5 ) (210.1 ) Deferred tax assets, net of valuation allowances 283.7 420.3 Deferred tax liabilities: Convertible debt (304.4 ) (606.7 ) Intangible assets (66.6 ) (147.5 ) Other (18.3 ) (6.3 ) Deferred tax liabilities (389.3 ) (760.5 ) Net deferred tax liability $ (105.6 ) $ (340.2 ) Reported as: Non-current deferred tax assets $ 100.2 $ 68.9 Non-current deferred tax liability (205.8 ) (409.1 ) Net deferred tax liability $ (105.6 ) $ (340.2 ) In assessing whether it is more likely than not that deferred tax assets will be realized, the Company considers all available evidence, both positive and negative, including its recent cumulative earnings experience and expectations of future available taxable income of the appropriate character by taxing jurisdiction, tax attribute carryback and carryforward periods available to them for tax reporting purposes, and prudent and feasible tax planning strategies. The Company had federal, state and foreign NOL carryforwards with an estimated tax effect of $ 101.1 million available at March 31, 2018 . The federal and state NOL carryforwards expire at various times between 2018 and 2037 . The Company believes that it is more likely than not that the benefit from certain foreign and state NOL carryforwards will not be realized. In recognition of this risk, at March 31, 2018 , the Company has provided a valuation allowance of $ 74.6 million . The Company also has state tax credits with an estimated tax effect of $ 96.3 million available at March 31, 2018 . These state tax credits expire at various times between 2018 and 2037 . The Company believes that it is more likely than not that the full benefit from these state tax credits will not be realized, and therefore has provided a valuation allowance of $ 96.3 million . The Company has capital loss carryforwards with an estimated tax effect of $ 10.6 million available at March 31, 2018 . These capital loss carryforwards begin to expire in 2020 . The Company believes that it is more likely than not that the full benefit from these capital losses will not be realized, and therefore has provided a valuation allowance of $ 10.4 million . The Company no longer maintains a U.S. foreign tax credit carryforward, alternative minimum tax credit carryforward, or credit for increasing research activities credit carryforwards, as all of these credits are expected to be utilized as a result of the one-time transition tax in the United States as of March 31, 2018. The Company had refundable foreign tax credits of $49.2 million available at March 31, 2018 . In addition, the Company had $ 20.0 million of withholding tax credits that expire at various times between 2022 and 2024 in foreign jurisdictions. The Company believes it is more likely than not that the benefit from these credits will not be fully realized and has provided a valuation allowance of $ 20.0 million . During the year ended March 31, 2016, the H.R. 2029 "Protecting Americans from Tax Hikes Act of 2015" was signed into law which extended certain business tax provisions through December 31, 2019, including IRC section 954(c)(6) dealing with the application of Subpart F to certain inter-company payments among controlled foreign corporations. The expiration of section 954(c)(6) and the other expired provisions could have a material impact on the Company's consolidated results of operations subsequent to the year ended March 31, 2020. The Company recognizes interest and penalties related to unrecognized tax benefits through income tax expense. The Company is subject to income taxes in the U.S. and numerous foreign jurisdictions. The Company files U.S. federal, U.S. state, and foreign income tax returns. For U.S. federal, and in general for U.S. state tax returns, the fiscal 2005 and later tax years remain effectively open for examination by tax authorities. For foreign tax returns, the Company is generally no longer subject to income tax examinations for years prior to fiscal 2007. Significant judgment is required in evaluating the Company's uncertain tax positions and determining its provision for income taxes. Although the Company believes that it has appropriately reserved for its uncertain tax positions, no assurance can be given that the final tax outcome of these matters will not be different than expectations. The Company will adjust these reserves in light of changing facts and circumstances, such as the closing of a tax audit, the refinement of an estimate, the closing of a statutory audit period or changes in applicable tax law. To the extent that the final tax outcome of these matters is different than the amounts recorded, such differences would impact the provision for income taxes in the period in which such determination is made. The provision for income taxes includes the impact of reserve provisions and changes to the reserves that are considered appropriate, as well as related net interest. The Company recognizes liabilities for anticipated tax audit issues in the U.S. and other domestic and international tax jurisdictions based on its estimate of whether, and the extent to which, additional tax payments are more likely than not. The Company believes that it has appropriate support for the income tax positions taken and to be taken on its tax returns and that its accruals for tax liabilities are adequate for all open years based on an assessment of many factors including past experience and interpretations of tax laws applied to the facts of each matter. The Company believes it maintains appropriate reserves to offset any potential income tax liabilities that may arise upon final resolution of matters for open tax years. If such reserve amounts ultimately prove to be unnecessary, the resulting reversal of such reserves could result in tax benefits being recorded in the period the reserves are no longer deemed necessary. If such amounts prove to be less than an ultimate assessment, a future charge to expense would be recorded in the period in which the assessment is determined. The following table summarizes the activity related to the Company's gross unrecognized tax benefits from April 1, 2015 to March 31, 2018 (amounts in millions): Year Ended March 31, 2018 2017 2016 Beginning balance $ 398.5 $ 220.7 $ 170.7 Increases related to acquisitions — 193.3 46.2 Decreases related to settlements with tax authorities (0.1 ) (11.7 ) (8.0 ) Decreases related to statute of limitation expirations (10.9 ) (7.6 ) (4.6 ) Increases related to current year tax positions 30.3 26.3 16.4 Increases (decreases) related to prior year tax positions 18.2 (22.5 ) — Ending balance $ 436.0 $ 398.5 $ 220.7 As of March 31, 2018 , the Company had accrued approximately $ 12.9 million related to the potential payment of interest on the Company's uncertain tax positions. As of March 31, 2017 , the Company had accrued approximately $ 9.4 million related to the potential payment of interest on the Company's uncertain tax positions. Interest was included in the provision for income taxes. The Company has accrued for approximately $ 67.9 million and $66.1 million in penalties related to its uncertain tax positions related primarily to its international locations as of March 31, 2018 and March 31, 2017 , respectively. Interest and penalties charged to operations during the years ended March 31, 2018 , 2017 and 2016 related to the Company's uncertain tax positions were $ 5.4 million , $ 5.8 million and $ 1.7 million , respectively. The total amount of gross unrecognized tax benefits was $436.0 million and $398.5 million as of March 31, 2018 and 2017, respectively, and is estimated to impact the Company’s effective tax rate, if recognized. We estimate that our unrecognized tax benefits as of March 31, 2018 could possibly decrease by approximately $20.0 million in the next 12 months. Positions that may be resolved include various U.S. and non-U.S. matters.

Debt and Credit Facility

Debt and Credit Facility	12 Months Ended
	Mar. 31, 2018
Convertible Debt [Abstract]
Debt and Credit Facility	Debt and Credit Facility Debt obligations included in the consolidated balance sheets consisted of the following (in millions): Coupon Interest Rate Effective Interest Rate Fair Value of Liability Component at Issuance (1) March 31, 2018 2017 Senior Indebtedness Credit Facility $ — $ — Senior Subordinated Convertible Debt - Principal Outstanding 2017 Senior Debt, maturing February 15, 2027 (2017 Senior Debt) 1.625% 6.0% $1,396.3 $ 2,070.0 $ 2,070.0 2015 Senior Debt, maturing February 15, 2025 (2015 Senior Debt) 1.625% 5.9% 1,160.1 1,725.0 1,725.0 Junior Subordinated Convertible Debt - Principal Outstanding 2017 Junior Debt, maturing February 15, 2037 (2017 Junior Debt) 2.250% 7.4% 321.1 686.3 575.0 2007 Junior Debt, maturing December 15, 2037 (2007 Junior Debt) 2.125% 9.1% — — 143.8 Total Convertible Debt 4,481.3 4,513.8 Gross long-term debt including current maturities 4,481.3 4,513.8 Less: Debt discount (2) (1,372.9 ) (1,516.5 ) Less: Debt issuance costs (3) (40.1 ) (46.8 ) Net long-term debt including current maturities 3,068.3 2,950.5 Less: Current maturities (4) (1,309.9 ) (50.0 ) Net long-term debt $ 1,758.4 $ 2,900.5 (1) As each of the convertible instruments may be settled in cash upon conversion, for accounting purposes, they were bifurcated into a liability component and an equity component, which are both initially recorded at fair value. The amount allocated to the equity component is the difference between the principal value of the instrument and the fair value of the liability component at issuance. The resulting debt discount is being amortized to interest expense at the respective effective interest rate over the contractual term of the debt. (2) The unamortized discount includes the following (in millions): March 31, 2018 2017 2017 Senior Debt $ (616.3 ) $ (667.5 ) 2015 Senior Debt (400.3 ) (446.6 ) 2017 Junior Debt (356.3 ) (309.3 ) 2007 Junior Debt — (93.1 ) Total unamortized discount $ (1,372.9 ) $ (1,516.5 ) (3) Debt issuance costs include the following (in millions): March 31, 2018 2017 Senior Credit Facility $ (5.9 ) $ (8.5 ) 2017 Senior Debt (16.1 ) (17.6 ) 2015 Senior Debt (14.8 ) (16.6 ) 2017 Junior Debt (3.3 ) (3.4 ) 2007 Junior Debt — (0.7 ) Total debt issuance costs $ (40.1 ) $ (46.8 ) (4) Current maturities include the full balance of the 2015 Senior Debt as of March 31, 2018 and the full balance of the 2007 Junior Debt as of March 31, 2017. Ranking of Indebtedness - The Senior Subordinated Convertible Debt and Junior Subordinated Convertible Debt (collectively, the Convertible Debt) are unsecured obligations which are subordinated in right of payment to the amounts outstanding under the Company's Credit Facility. The Junior Subordinated Convertible Debt is expressly subordinated in right of payment to any existing and future senior debt of the Company (including the Credit Facility and the Senior Subordinated Convertible Debt) and is structurally subordinated in right of payment to the liabilities of the Company's subsidiaries. The Senior Subordinated Convertible Debt is subordinated to the Credit Facility; ranks senior to the Company's indebtedness that is expressly subordinated in right of payment, including the Junior Subordinated Convertible Debt; ranks equal in right of payment to any of the Company's unsubordinated indebtedness that does not provide that it is senior to the Senior Subordinated Convertible Debt; ranks junior in right of payment to any of the Company's secured, unsubordinated indebtedness to the extent of the value of the assets securing such indebtedness; and ranks junior to all indebtedness and other liabilities of the Company's subsidiaries. Summary of Conversion Features - Each series of Convertible Debt is convertible, subject to certain conditions, into cash, shares of the Company's common stock or a combination thereof, at the Company's election, at specified Conversion Rates (see table below), adjusted for certain events including the declaration of cash dividends. Until the three-months immediately preceding the maturity date of the applicable series of Convertible Debt, each series of Convertible Debt is convertible only upon the occurrence of (1) such time as the closing price of the Company's common stock exceeds the Conversion Price (see table below) by 130% for 20 days (whether or not consecutive) during a period of 30 consecutive trading days ending on the last trading day of the immediately preceding fiscal quarter or (2) during the 5 business day period after any 10 consecutive trading day period, or the measurement period, in which the trading price per $1,000 principal amount of notes for each trading day of the measurement period was less than 98% of the product of the last reported sale price of the Company's common stock and the conversion rate on each such trading day or (3) upon the occurrence of certain corporate events specified in the indenture of such series of Convertible Debt. In addition, for each series, if at the time of conversion the applicable price of the Company's common stock exceeds the applicable Conversion Price at such time, the applicable Conversion Rate will be increased by up to an additional maximum incremental shares rate, as determined pursuant to a formula specified in the indenture for the applicable series of Convertible Debt, and as adjusted for cash dividends paid since the issuance of such series of Convertible Debt. However, in no event will the applicable Conversion Rate exceed the applicable Maximum Conversion Rate specified in the indenture for the applicable series of Convertible Debt (see table below). The following table sets forth the applicable Conversion Rates adjusted for dividends declared since issuance of such series of Convertible Debt and the applicable Incremental Share Factors and Maximum Conversion Rates as adjusted for dividends paid since the applicable issuance date: Dividend adjusted rates as of March 31, 2018 Conversion Rate, adjusted Approximate Conversion Price, adjusted Incremental Share Factor, adjusted Maximum Conversion Rate, adjusted 2017 Senior Debt 10.1162 $ 98.85 5.0581 14.4156 2015 Senior Debt 15.7755 $ 63.39 7.8878 22.0857 2017 Junior Debt 10.2969 $ 97.12 5.1485 14.4156 As of March 31, 2018 , the holders of the 2015 Senior Debt have the right to convert their debentures between April 1, 2018 and June 30, 2018 because the Company's common stock price has exceeded the Conversion Price by 130% for the specified period of time during the quarter ended March 31, 2018 . As of March 31, 2018 , the 2015 Senior Debt had a value if converted above par of $1,141.7 million . The 2015 Senior Debt is included in the current portion of long-term debt. The Company may not redeem any series of Convertible Debt prior to the relevant maturity date and no sinking fund is provided for any series of Convertible Debt. Upon the occurrence of a fundamental change as defined in the applicable indenture of such series of Convertible Debt, holders of such series may require the Company to purchase all or a portion of their Convertible Debt for cash at a price equal to 100% of the principal amount plus any accrued and unpaid interest. Interest expense related to convertible debt includes the following (in millions): Year Ended March 31, 2018 2017 2016 Debt issuance amortization $ 3.5 $ 2.1 $ 1.8 Amortization of debt discount - non cash interest expense 106.1 56.1 48.0 Coupon interest expense 77.3 44.5 40.2 Total $ 186.9 $ 102.7 $ 90.0 The remaining period over which the unamortized debt discount will be recognized as non-cash interest expense is 8.88 years , 6.88 years , and 18.88 years for the 2017 Senior Debt , 2015 Senior Debt and 2017 Junior Debt , respectively. Issuances and Settlements In November 2017, the Company called $14.6 million in principal value of the remaining outstanding 2007 Junior Debt with an effective date of December 15, 2017 for which substantially all holders submitted requests to convert. Prior to the call, conversion requests were received in both the second and third quarters of fiscal 2018. Total conversions for fiscal 2018 were for a principal amount of $32.5 million for which the Company settled the principal amount in cash and issued 0.5 million shares of its common stock in respect of the conversion value in excess of the principal amount for the conversions occurring prior to the call notice and $41.0 million in cash for the conversion value in excess of the principal amount for the conversion requests received after the call notice. A loss on total conversions was recorded for $2.2 million . The 2007 Junior Debt was classified as a current liability on the consolidated balance sheet as of March 31, 2017. In June 2017, the Company exchanged in privately negotiated transactions $111.3 million aggregate principal amount of its 2007 Junior Debt for (i) $111.3 million principal amount of 2017 Junior Debt with a market value of $119.3 million plus (ii) the issuance of 3.2 million shares of the Company's common stock with a value of $254.6 million , of which $56.3 million was allocated to the fair value of the liability and $321.1 million was allocated to the reacquisition of the equity component for total consideration of $374.0 million . The transaction resulted in a loss on settlement of the 2007 Junior Debt of approximately $13.8 million , which represented the difference between the fair value of the liability component at time of repurchase and the sum of the carrying values of the debt component and any unamortized debt issuance costs. The debt discount on the new 2017 Junior Debt was the difference between the par value and the fair value of the debt resulting in a debt discount of $55.1 million which will be amortized to interest expense using the effective interest method over the term of the debt. In February 2017, the Company issued the 2017 Senior Debt and 2017 Junior Debt for net proceeds of $2,043.6 million and $567.7 million , respectively. In connection with the issuance of these instruments, the Company incurred issuance costs of $33.7 million , of which $17.8 million and $3.4 million was recorded as debt issuance costs related to the 2017 Senior Debt and 2017 Junior Debt, respectively, and will be amortized using the effective interest method over the term of the debt. The balance of $12.5 million in fees was recorded to equity. Interest on both instruments is payable semi-annually on February 15 and August 15 of each year. In February 2015, the Company issued the 2015 Senior Debt for net proceeds of approximately $1,694.7 million . In connection with the issuance, the Company incurred issuance costs of $30.3 million, of which $20.4 million was recorded as debt issuance costs and will be amortized using the effective interest method over the term of the debt. The balance of $9.9 million was recorded to equity. The Company utilized the proceeds from the issuances of the 2017 Senior Debt, 2017 Junior Debt, and 2015 Senior Debt to reduce amounts borrowed under its Credit Facility and to settle a portion of the 2007 Junior Debt in privately negotiated transactions. In February 2017 and February 2015, the Company settled $431.3 million and $575.0 million , respectively, in aggregate principal of its 2007 Junior Debt. The 2015 repurchase consisted solely of cash. In February 2017 , the Company used cash of $431.3 million and an aggregate of 12.0 million in shares of the Company's common stock valued at $862.7 million for total consideration of $1,293.9 million to settle $431.3 million of the 2007 Junior Debt, of which $188.0 million was allocated to the liability component and $1,105.9 million was allocated to the equity component. In addition, in February 2017, there was an inducement fee of $5.0 million which was recorded in the consolidated statements of income in loss on settlement of convertible debt. The consideration transferred in February 2015 was $1,134.6 million , of which $238.3 million was allocated to the liability component and $896.3 million was allocated to the equity component. In the case of both settlements of the 2007 Junior Debt, the consideration was allocated to the liability and equity components using the equivalent rate that reflected the borrowing rate for a similar non-convertible debt prior to the retirement. The transactions resulted in a loss on settlement of convertible debt of approximately $43.9 million and $50.6 million in fiscal 2017 and fiscal 2015, respectively, which represented, in each case, the difference between the fair value of the liability component at time of repurchase and the sum of the carrying values of the debt component and any unamortized debt issuance costs. Credit Facility The Company maintains a credit facility which is available until February 4, 2020 (the "Credit Agreement") which was amended subsequent to the end of fiscal 2018. See Note 23 for more details on subsequent events. At the beginning of the second quarter of fiscal 2018, the credit facility had a borrowing capacity of $2.774 billion comprised of two tranches; one tranche terminating in 2018 (the "2018 Tranche") and one tranche terminating in 2020 (the "2020 Tranche"). During the second quarter of fiscal 2018, the Company terminated the 2018 Tranche and in connection with such termination increased the commitments for the 2020 Tranche in each of the three months ended September 30, 2017 and December 31, 2017. In November 2017, the Company entered into an augmenting lender supplement which added a new lender to the Credit Agreement with a 2020 multicurrency tranche commitment. As of March 31, 2018 , the 2020 Tranche commitment under the credit facility was $3.122 billion . The financial covenants include, among others, limits on the Company's consolidated senior ratio and total leverage ratio. The maximum Total Leverage Ratio (capitalized terms not otherwise defined in this Form 10-K have the meaning of the defined terms in the applicable agreements) cannot exceed 5.00 to 1.00 and is calculated as Consolidated Total Indebtedness, excluding the Junior Debt up to a $700 million maximum, to Consolidated EBIDTA for a period of four quarters. The Total Leverage Ratio may be temporarily increased to 5.50 to 1.00 for a period of four consecutive quarters in conjunction with a Permitted Acquisition occurring during the first four quarters following the acquisition. The Total Leverage Ratio then decreases to 5.25 to 1.00 for three consecutive quarters, finally returning to the stated 5.00 to 1.00 Total Leverage Ratio after a period of seven consecutive fiscal periods. The Company can elect to use this special feature, also referred to as an Adjusted Covenant Period, not more than one time from and after February 8, 2017, the effective date of the February 2017 amendment (discussed below), and may elect to terminate an Adjusted Covenant Period prior to the end of the Adjusted Covenant Period. The Credit Facility also requires that the Senior Leverage Ratio not exceed 3.50 to 1.00, which is calculated as Consolidated Senior Indebtedness to Consolidated EBIDTA for four consecutive quarters. The Company is also required to comply with an Interest Coverage Ratio of at least 3.50 to 1.00, measured quarterly. In June 2017, in connection with the settlement of the 2007 Junior Debt, the Company amended the Credit Agreement to (i) extend the time period during which the Company is permitted to repurchase, redeem or exchange the 2007 Junior Debt and (ii) amend the maximum total leverage ratio covenant to extend the time period for permitted refinancings or exchanges of the 2007 Junior Debt that may be excluded from the calculation of the ratio, subject to certain conditions. The Credit Agreement has a $ 125 million foreign currency sublimit, a $ 25 million letter of credit sublimit and a $25 million swingline loan sublimit. The Company has the option to obtain additional tranche commitments or additional indebtedness as long as, on a proforma basis, the Senior Leverage Ratio is equal to or less than 2.50 to 1.00. In February 2017, the Company used $1,682.5 million of the proceeds from the issuance of the 2017 Senior Debt and 2017 Junior Debt to pay off the entire balance under the revolving credit facility. In connection with the February 2017 amendment to the Credit Agreement, the Company incurred $2.1 million of issuance fees which will be amortized over the term of the facility and for which the balance is recorded net of any outstanding Credit Facility balance. At March 31, 2018 and 2017, there were no outstanding borrowings under the revolving credit facility and no letters of credit outstanding under the Credit Agreement. The Company's obligations under the Credit Agreement are guaranteed by certain of its subsidiaries meeting materiality thresholds set forth in the Credit Agreement. To secure the Company's obligations under the Credit Agreement, the Company and its domestic subsidiaries are required to pledge the equity securities of certain of their respective material subsidiaries, subject to certain exceptions and limitations. In addition, in connection with the February 2017 amendment, the Company and the guarantor subsidiaries granted a security interest in substantially all of their personal property to secure the obligations under the Credit Agreement. The loans under the Credit Agreement bear interest, at the Company's option, at the base rate plus a spread of 0.25% to 1.25% or an adjusted LIBOR rate (based on one, two, three, or six-month interest periods) plus a spread of 1.25% to 2.25% , in each case with such spread being determined based on the consolidated leverage ratio for the preceding four fiscal quarters (in the case of the 2018 tranche revolving loans) or the consolidated senior leverage ratio (in the case of the 2020 tranche revolving loans). The base rate means the highest of JPMorgan Chase Bank, N.A.'s prime rate, the federal funds rate plus a margin equal to 0.50% and the adjusted LIBOR rate for a 1-month interest period plus a margin equal to 1.00% . Swingline loans accrue interest at a per annum rate based on the base rate plus the applicable margin for base rate loans. Base rate loans may only be made in U.S. Dollars. The Company is also obligated to pay other customary administration fees and letter of credit fees for a credit facility of this size and type. Interest is due and payable in arrears quarterly for loans bearing interest at the base rate and at the end of an interest period (or at each three-month interval in the case of loans with interest periods greater than three months) in the case of loans bearing interest at the adjusted LIBOR rate. Interest expense related to the credit agreement was approximately $9.7 million in fiscal 2018 , approximately $42.9 million in fiscal 2017 and approximately $ 18.9 million in fiscal 2016 . Principal, together with all accrued and unpaid interest, is due and payable on the respective tranche maturity date, which is February 4, 2020 for the 2020 tranche revolving loans. The Company pays a quarterly commitment fee on the available but unused portion of its line of credit which is calculated on the average daily available balance during the period. The Company may prepay the loans and terminate the commitments, in whole or in part, at any time without premium or penalty, subject to certain conditions including minimum amounts in the case of commitment reductions and reimbursement of certain costs in the case of prepayments of LIBOR loans. The Credit Agreement contains customary affirmative and negative covenants, including covenants that limit or restrict the Company and its subsidiaries' ability to, among other things, incur subsidiary indebtedness, grant liens, merge or consolidate, dispose of assets, make investments, make acquisitions, enter into certain transactions with affiliates, pay dividends or make distributions, repurchase stock, enter into restrictive agreements and enter into sale and leaseback transactions, in each case subject to customary exceptions for a credit facility of this size and type. The Company is also required to maintain compliance with a senior leverage ratio, a total leverage ratio and an interest coverage ratio, all measured quarterly and calculated on a consolidated bases. At March 31, 2018 , the Company was in compliance with these financial covenants. The Credit Agreement includes customary events of default that include, among other things, non-payment defaults, inaccuracy of representations and warranties, covenant defaults, cross default to material indebtedness, bankruptcy and insolvency defaults, material judgment defaults, ERISA defaults and a change of control default. The occurrence of an event of default could result in the acceleration of the obligations under the Credit Agreement. Under certain circumstances, a default interest rate will apply on all obligations during the existence of an event of default under the Credit Agreement at a per annum rate equal to 2.00% above the applicable interest rate for any overdue principal and 2.00% above the rate applicable for base rate loans for any other overdue amounts.

Contingencies

Contingencies	12 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Contingencies	Contingencies In the ordinary course of the Company's business, it is exposed to various liabilities as a result of contracts, product liability, customer claims and other matters. Additionally, the Company is involved in a limited number of legal actions, both as plaintiff and defendant. Consequently, the Company could incur uninsured liability in any of those actions. The Company also periodically receives notifications from various third parties alleging infringement of patents or other intellectual property rights, or from customers requesting reimbursement for various costs. With respect to pending legal actions to which the Company is a party and other claims, although the outcomes are generally not determinable, the Company believes that the ultimate resolution of these matters will not have a material adverse effect on its financial position, cash flows or results of operations. Litigation and disputes relating to the semiconductor industry are not uncommon, and the Company is, from time to time, subject to such litigation and disputes. As a result, no assurances can be given with respect to the extent or outcome of any such litigation or disputes in the future. As a result of its acquisition of Atmel, which closed April 4, 2016, the Company became involved with the following legal matters: In re: Continental Airbag Products Liability Litigation . On May 11, 2016, an Amended and Consolidated Class Action Complaint ("Complaint") was filed in the United States District Court for the Southern District of Florida (Miami Division) against Atmel, Continental Automotive Systems, Inc., Honda Motor Co., Ltd. and an affiliate, and Daimler AG and an affiliate. The Complaint included claims arising under federal law and Florida, California, New Jersey, Michigan and Louisiana state law and alleged that class members unknowingly purchased or leased vehicles containing defective airbag control units (incorporating allegedly defective application specific integrated circuits manufactured by the Company's Atmel subsidiary between 2006 and 2010), and thereby suffered financial harm, including a loss in the value of their purchased or leased vehicles. The plaintiffs were seeking, individually and on behalf of a putative class, unspecified compensatory and exemplary damages, statutory penalties, pre- and post-judgment interest, attorneys' fees, and injunctive and other relief. The Company's Atmel subsidiary contested plaintiffs' claims vigorously, and on May 23, 2017 the case was ordered to be dismissed. Continental Claim ICC Arbitration. On December 29, 2016, Continental Automotive GmbH ("Continental") filed a Request for Arbitration with the ICC, naming as respondents the Company's subsidiaries Atmel Corporation, Atmel SARL, Atmel Global Sales Ltd., and Atmel Automotive GmbH (collectively, “Atmel”). The Request alleges that a quality issue affecting Continental airbag control units in certain recalled vehicles stems from allegedly defective Atmel application specific integrated circuits (“ASICs”). The Continental airbag control units, ASICs and vehicle recalls were also at issue in In re: Continental Airbag Products Liability Litigation , described above. Continental seeks to recover from Atmel all related costs and damages incurred as a result of the vehicle manufacturers’ airbag control unit-related recalls, currently alleged to be $227.7 million . The Company's Atmel subsidiaries intend to defend this action vigorously. Southern District of New York Action by LFoundry Rousset ("LFR") and LFR Employees . On March 4, 2014, LFR and Jean-Yves Guerrini, individually and on behalf of a putative class of LFR employees, filed an action in the United States District Court for the Southern District of New York (the "District Court") against the Company's Atmel subsidiary, French subsidiary, Atmel Rousset S.A.S. ("Atmel Rousset"), and LFoundry GmbH ("LF"), LFR's German parent. The case purports to relate to Atmel Rousset's June 2010 sale of its wafer manufacturing facility in Rousset, France to LF, and LFR's subsequent insolvency, and later liquidation, more than three years later. The District Court dismissed the case on August 21, 2015, and the United States Court of Appeals for the Second Circuit affirmed the dismissal on June 27, 2016. On July 25, 2016, the plaintiffs filed a notice of appeal from the District Court's June 27, 2016 denial of their motion for relief from the dismissal judgment. On May 19, 2017, the United States Court of Appeals for the Second Circuit affirmed the June 27, 2016 order dismissing the case. Individual Labor Actions by former LFR Employees . In the wake of LFR's insolvency and liquidation, over 500 former employees of LFR have filed individual labor actions against Atmel Rousset in a French labor court. The Company's Atmel Rousset subsidiary believes that each of these actions is entirely devoid of merit, and, further, that any assertion by any of the Claimants of a co-employment relationship with the Atmel Rousset subsidiary is based substantially on the same specious arguments that the Paris Commercial Court summarily rejected in 2014 in related proceedings. The Company's Atmel Rousset subsidiary therefore intends to defend vigorously against each of these claims. Additionally, complaints have been filed in a regional court in France on behalf of the same group of employees against Microchip Technology Rousset, Atmel Switzerland Sarl, Atmel Corporation and Microchip Technology Incorporated alleging that the sale of the Atmel Rousset production unit to LFoundry GmbH was fraudulent and should be voided. These claims are based largely on the same specious arguments as listed in the Southern District of New York Action listed above. The defendant entities therefore intend to defend vigorously against these claims. The Company accrues for claims and contingencies when losses become probable and reasonably estimable. As of the end of each applicable reporting period, the Company reviews each of its matters and, where it is probable that a liability has been or will be incurred, the Company accrues for all probable and reasonably estimable losses. Where the Company can reasonably estimate a range of losses it may incur regarding such a matter, the Company records an accrual for the amount within the range that constitutes its best estimate. If the Company can reasonably estimate a range but no amount within the range appears to be a better estimate than any other, the Company uses the amount that is the low end of such range. As of March 31, 2018 , the Company's estimate of the aggregate potential liability that is possible but not probable is approximately $100 million in excess of amounts accrued. The Company's technology license agreements generally include an indemnification clause that indemnifies the licensee against liability and damages (including legal defense costs) arising from any claims of patent, copyright, trademark or trade secret infringement by the Company's proprietary technology. The terms of these indemnification provisions approximate the terms of the outgoing technology license agreements, which are typically perpetual unless terminated by either party for breach. The possible amount of future payments the Company could be required to make based on agreements that specify indemnification limits, if such indemnifications were required on all of these agreements, is approximately $160.5 million . There are some licensing agreements in place that do not specify indemnification limits. As of March 31, 2018 , the Company had not recorded any liabilities related to these indemnification obligations and the Company believes that any amounts that it may be required to pay under these agreements in the future will not have a material adverse effect on its financial position, cash flows or results of operations.

Stock Repurchase Activity

Stock Repurchase Activity	12 Months Ended
	Mar. 31, 2018
Equity [Abstract]
Stock Repurchase Activity	Stock Repurchase Activity In December 2007 , the Company announced that its Board of Directors had authorized the repurchase of up to 10.0 million shares of its common stock in the open market or in privately negotiated transactions. As of March 31, 2015, the Company had repurchased 7.5 million shares under this authorization for $ 234.7 million . In May 2015, the Company's Board of Directors authorized an increase to the existing share repurchase program to 20.0 million shares of common stock from the approximately 2.5 million shares remaining under the prior authorization. During fiscal 2016, the Company repurchased 8.6 million shares under this authorization for $ 363.8 million . In January 2016, the Company's Board of Directors authorized an increase to the existing share repurchase program to 15.0 million shares of common stock from the approximately 11.4 million shares remaining under the prior authorization. There were no repurchases of common stock during fiscal 2018 and fiscal 2017 . There is no expiration date associated with this repurchase program. As of March 31, 2018 , approximately 18.2 million shares remained as treasury shares with the balance of the shares being used to fund share issuance requirements under the Company's equity incentive plans.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Mar. 31, 2018
Retirement Benefits [Abstract]
Employee Benefit Plans	Employee Benefit Plans Defined Benefit Plans In connection with its acquisition of Atmel, the Company assumed unfunded defined benefit pension plans that cover certain French and German employees. Plan benefits are provided in accordance with local statutory requirements. Benefits are based on years of service and employee compensation levels. Pension liabilities and charges are based upon various assumptions, updated annually, including discount rates, future salary increases, employee turnover, and mortality rates. The Company’s French pension plan provides for termination benefits paid to covered French employees only at retirement, and consists of approximately one to five months of salary. The Company's German pension plan provides for defined benefit payouts for covered German employees following retirement. The aggregate net pension expense relating to these two plans is as follows (amounts in millions): Year Ended March 31, 2018 2017 Service costs $ 2.2 $ 1.4 Interest costs 1.0 1.0 Amortization of actuarial loss 0.8 — Settlements — 0.5 Net pension period cost $ 4.0 $ 2.9 Interest costs and amortization of actuarial losses are recorded in the other (loss) income, net line item in the statements of income. The change in projected benefit obligation and the accumulated benefit obligation, were as follows (amounts in millions): Year Ended March 31, 2018 2017 Projected benefit obligation at the beginning of the year $ 50.4 $ 40.3 Service cost 2.2 1.4 Interest cost 1.0 1.0 Settlements — 0.5 Actuarial losses 0.7 7.9 Benefits paid (0.8 ) (0.4 ) Foreign currency exchange rate changes 7.5 (0.3 ) Projected benefit obligation at the end of the year $ 61.0 $ 50.4 Accumulated benefit obligation at the end of the year $ 55.5 $ 45.6 Weighted average assumptions Discount rate 1.73 % 1.82 % Rate of compensation increase 2.91 % 2.90 % The Company's pension liability represents the present value of estimated future benefits to be paid. The discount rate is based on the quarterly average yield for Euros treasuries with a duration of 30 years, plus a supplement for corporate bonds (Euros, AA rating). Net actuarial losses, which are included in accumulated other comprehensive loss in the Company's consolidated balance sheets, will be recognized as a component of net periodic cost over the average remaining service period. As the defined benefit plans are unfunded, the liability recognized on the Company's consolidated balance sheet as of March 31, 2018 was $61.0 million of which $0.9 million is included in accrued liabilities and $60.1 million is included in other long-term liabilities. The liability recognized on the Company's consolidated balance sheet as of March 31, 2017 was $50.4 million of which $0.7 million is included in accrued liabilities and $49.7 million is included in other long-term liabilities. Future estimated expected benefit payments for fiscal year 2019 through 2028 are as follows (amounts in millions): Fiscal Year Ending March 31, Expected Benefit Payments 2019 $ 0.9 2020 0.9 2021 1.2 2022 1.6 2023 1.6 2024 through 2028 10.6 Total $ 16.8 The Company's net periodic pension cost for fiscal 2019 is expected to be approximately $3.1 million . In connection with the acquisition of SMSC in August 2012, the Company assumed an unfunded Supplemental Executive Retirement Plan ("SERP"), which provides former SMSC senior management with retirement, disability and death benefits. An amendment to the SERP was executed on November 3, 2009, freezing the benefit level for existing participants as of February 28, 2010 and closing the SERP to new participants. As of March 31, 2018 , the projected benefit obligation is $ 4.0 million . Annual benefit payments and contributions under this plan are expected to be approximately $ 0.4 million in fiscal 2019 and approximately $ 3.5 million cumulatively in fiscal 2020 through fiscal 2028. Defined Contribution Plans The Company maintains a contributory profit-sharing plan for its domestic employees meeting certain eligibility and service requirements. The plan qualifies under Section 401(k) of the Internal Revenue Code of 1986, as amended, and allows employees to contribute up to 60% of their base salary, subject to maximum annual limitations prescribed by the IRS. The Company has a discretionary matching contribution program. All matches are provided on a quarterly basis and require the participant to be an active employee at the end of the applicable quarter. During fiscal 2018 , 2017 and 2016 , the Company's matching contributions to the plan totaled $ 8.8 million , $ 8.2 million and $ 4.4 million , respectively. The Company's 2001 Employee Stock Purchase Plan (the 2001 Purchase Plan) became effective on March 1, 2002 . Under the 2001 Purchase Plan, eligible employees of the Company may purchase shares of common stock at semi-annual intervals through periodic payroll deductions. The purchase price in general will be 85% of the lower of the fair market value of the common stock on the first day of the participant's entry date into the offering period or of the fair market value on the semi-annual purchase date. Depending upon a participant's entry date into the 2001 Purchase Plan, purchase periods under the 2001 Purchase Plan consist of overlapping periods of either 24 , 18 , 12 or 6 months in duration. In May 2003 and August 2003 , the Company's Board and stockholders, respectively, each approved an annual automatic increase in the number of shares reserved under the 2001 Purchase Plan. The automatic increase took effect on January 1, 2005 , and on each January 1 thereafter during the term of the plan, and is equal to the lesser of (i) 1,500,000 , (ii) one half of one percent ( 0.5% ) of the then outstanding shares of the Company's common stock, or (iii) such lesser amount as is approved by Board of Directors . Upon the approval of the Board of Directors, there were no shares added under the 2001 Purchase Plan on January 1, 2018 based on the automatic increase provision. On January 1, 2017 and 2016, an additional 1,077,150 shares and 1,017,492 shares, respectively, were reserved under the 2001 Purchase Plan based on the automatic increase. Since the inception of the 2001 Purchase Plan, 13,372,504 shares of common stock have been reserved for issuance and 7,895,340 shares have been issued under this purchase plan. During fiscal 1995, a purchase plan was adopted for employees in non-U.S. locations. Such plan provided for the purchase price per share to be 100% of the lower of the fair market value of the common stock at the beginning or end of the semi-annual purchase plan period. Effective May 1, 2006, the Company's Board of Directors approved a purchase price per share equal to 85% of the lower of the fair market value of the common stock at the beginning or end of the semi-annual purchase plan period. On May 1, 2006, the Company's Board of Directors approved an annual automatic increase in the number of shares reserved under the plan. The automatic increase took effect on January 1, 2007, and on each January 1 thereafter during the term of the plan, and is equal to one tenth of one percent ( 0.1% ) of the then outstanding shares of the Company's common stock. Upon the approval of the Board of Directors, there were no shares added under the plan on January 1, 2018 based on the automatic increase provision. On January 1, 2017 and 2016, an additional 215,430 shares and 203,498 shares, respectively, were reserved under the plan based on the automatic increase. Since the inception of this purchase plan, 1,919,213 shares of common stock have been reserved for issuance and 1,347,972 shares have been issued under this purchase plan. Effective January 1, 1997 , the Company adopted a non-qualified deferred compensation arrangement. This plan is unfunded and is maintained primarily for the purpose of providing deferred compensation for a select group of highly compensated employees as defined in ERISA Sections 201, 301 and 401. There are no Company matching contributions made under this plan. The Company has management incentive compensation plans which provide for bonus payments, based on a percentage of base salary, from an incentive pool created from operating profits of the Company, at the discretion of the Board of Directors. During fiscal 2018 , 2017 and 2016 , $ 48.1 million , $ 41.5 million and $ 19.1 million were charged against operations for these plans, respectively. The Company also has a plan that, at the discretion of the Board of Directors, provides a cash bonus to all employees of the Company based on the operating profits of the Company. During fiscal 2018 , 2017 and 2016 , $ 36.3 million , $ 28.2 million and $ 14.2 million , respectively, were charged against operations for this plan.

Share-Based Compensation

Share-Based Compensation	12 Months Ended
	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-Based Compensation	Share-Based Compensation Share-Based Compensation Expense The following table presents the details of the Company's share-based compensation expense (amounts in millions): Year Ended March 31, 2018 2017 2016 Cost of sales $ 13.8 (1) $ 18.7 (1) $ 8.3 (1) Research and development 42.5 46.8 32.0 Selling, general and administrative 36.9 62.6 31.1 Pre-tax effect of share-based compensation 93.2 128.1 71.4 Income tax benefit 28.3 (2) 44.2 (2) 23.0 Net income effect of share-based compensation $ 64.9 $ 83.9 $ 48.4 (1) During the year ended March 31, 2018 , $ 11.9 million of share-based compensation expense was capitalized to inventory, and $ 13.8 million of previously capitalized share-based compensation expense in inventory was sold. During the year ended March 31, 2017 , $ 11.3 million of share-based compensation expense was capitalized to inventory. The amount of share-based compensation included in cost of sales during fiscal 2017 included $ 14.5 million of previously capitalized share-based compensation expense in inventory that was sold and $ 4.2 million of share-based compensation expense related to the Company's acquisition of Atmel that was not previously capitalized to inventory. During the year ended March 31, 2016 , $ 7.9 million of share-based compensation expense was capitalized to inventory, and $ 8.3 million of previously capitalized share-based compensation expense in inventory was sold. (2) Amounts exclude excess tax benefits related to share-based compensation of $ 28.1 million and $ 25.0 million for the years ended March 31, 2018 and March 31, 2017 , respectively. The Company elected to early adopt ASU 2016-09 effective April 1, 2016. Prior to the adoption of ASU 2016-09, the Company recognized excess tax benefits related to share-based compensation in additional paid-in capital. Refer to Note 1 for additional information on the adoption of this standard. The amount of unearned share-based compensation currently estimated to be expensed in the remainder of fiscal 2019 through fiscal 2023 related to unvested share-based payment awards at March 31, 2018 is $ 169.6 million . The weighted average period over which the unearned share-based compensation is expected to be recognized is approximately 1.91 years. Atmel Acquisition-related Equity Awards In connection with the acquisition of Atmel, the Company assumed certain RSUs granted by Atmel. The assumed awards were measured at the acquisition date based on the estimated fair value, which was a total of $95.9 million . A portion of that fair value, $7.5 million , which represented the pre-acquisition vested service provided by employees to Atmel, was included in the total consideration transferred as part of the acquisition. As of the acquisition date, the remaining portion of the fair value of those awards was $88.4 million , representing post-acquisition share-based compensation expense that will be recognized as these employees provide service over the remaining vesting periods. Combined Incentive Plan Information RSU share activity under the 2004 Plan is set forth below: Number of Shares Weighted Average Grant Date Fair Value Nonvested shares at March 31, 2015 5,268,916 $ 34.15 Granted 2,479,729 38.91 Assumed upon acquisition 525,442 40.58 Forfeited (360,072 ) 38.20 Vested (1,606,273 ) 32.47 Nonvested shares at March 31, 2016 6,307,742 36.76 Granted 1,635,655 51.46 Assumed upon acquisition 2,059,524 46.57 Forfeited (722,212 ) 43.58 Vested (2,861,253 ) 38.60 Nonvested shares at March 31, 2017 6,419,456 42.06 Granted 1,267,536 77.26 Forfeited (279,051 ) 49.65 Vested (1,735,501 ) 38.00 Nonvested shares at March 31, 2018 5,672,440 $ 50.79 The total intrinsic value of RSUs which vested during the years ended March 31, 2018 , 2017 and 2016 was $ 146.0 million , $ 166.1 million and $ 72.1 million , respectively. The aggregate intrinsic value of RSUs outstanding at March 31, 2018 was $ 518.2 million , calculated based on the closing price of the Company's common stock of $91.36 per share on March 31, 2018. At March 31, 2018 , the weighted average remaining expense recognition period was 1.96 years. Stock option and stock appreciation right (SAR) activity under the Company's stock incentive plans in the three years ended March 31, 2018 is set forth below: Number of Shares Weighted Average Exercise Price per Share Outstanding at March 31, 2015 684,299 $ 28.41 Granted 244 41.09 Assumed upon acquisition 604,900 35.03 Exercised (221,987 ) 25.30 Forfeited or expired (153,948 ) 31.52 Outstanding at March 31, 2016 913,508 33.00 Exercised (437,906 ) 34.34 Forfeited or expired (42,485 ) 34.26 Outstanding at March 31, 2017 433,117 31.51 Exercised (131,666 ) 31.75 Forfeited or expired (17,111 ) 34.73 Outstanding at March 31, 2018 284,340 $ 31.21 The total intrinsic value of options and SARs exercised during the years ended March 31, 2018 , 2017 and 2016 was $ 7.4 million , $ 9.6 million and $ 4.7 million , respectively. This intrinsic value represents the difference between the fair market value of the Company's common stock on the date of exercise and the exercise price of each equity award. The aggregate intrinsic value of options and SARs outstanding at March 31, 2018 was $ 17.1 million . The aggregate intrinsic value of options and SARS exercisable at March 31, 2018 was $ 13.8 million . The aggregate intrinsic values were calculated based on the closing price of the Company's common stock of $91.36 per share on March 29, 2018. As of March 31, 2018 and March 31, 2017 , the number of option and SAR shares exercisable was 224,022 and 264,061 , respectively, and the weighted average exercise price per share was $ 29.96 and $ 29.59 , respectively. The weighted average fair values per share of stock options granted in the year ended March 31, 2016 was $ 8.85 . The fair values per share of stock options granted in the year ended March 31, 2016 were estimated utilizing the following assumptions: Year Ended March 31, 2016 Expected term (in years) 6.5 Volatility 29.50 % Risk-free interest rate 1.54 % Dividend yield 3.00 % There were no stock options granted in the years ended March 31, 2018 and March 31, 2017 .

Commitments

Commitments	12 Months Ended
	Mar. 31, 2018
Commitments [Abstract]
Commitments	Commitments The Company leases office space and transportation and other equipment under operating leases which expire at various dates through December 31, 2022. The future minimum lease commitments under these operating leases at March 31, 2018 were as follows (amounts in millions): Year Ending March 31, Amount 2019 $ 26.0 2020 18.7 2021 13.3 2022 10.2 2023 1.7 Thereafter — Total minimum payments $ 69.9 The terms of the leases do not contain significant restriction provisions and usually contain standard rent escalation clauses as well as options for renewal. Rental expense under operating leases totaled $ 30.0 million , $ 35.4 million and $ 23.3 million for fiscal 2018 , 2017 and 2016 , respectively. Commitments for construction or purchase of property, plant and equipment totaled $ 144.4 million as of March 31, 2018 , all of which will be due within the next year. Other purchase obligations and commitments totaled approximately $ 101.6 million of outstanding purchase commitments with the Company's wafer foundries for delivery in fiscal 2019 .

Geographic and Segment Informat

Geographic and Segment Information	12 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Geographic and Segment Information	Geographic and Segment Information The Company's reporting segments include semiconductor products and technology licensing. The Company does not allocate operating expenses, interest income, interest expense, other income or expense, or provision for or benefit from income taxes to these segments for internal reporting purposes, as the Company does not believe that allocating these expenses is beneficial in evaluating segment performance. Additionally, the Company does not allocate assets to segments for internal reporting purposes as it does not manage its segments by such metrics. The following table represents revenues and gross profit for each segment (amounts in millions): Years ended March 31, 2018 2017 2016 Net Sales Gross Profit Net Sales Gross Profit Net Sales Gross Profit Semiconductor products $ 3,876.0 $ 2,315.9 $ 3,316.6 $ 1,666.0 $ 2,084.2 $ 1,116.4 Technology licensing 104.8 104.8 91.2 91.2 89.1 89.1 Total $ 3,980.8 $ 2,420.7 $ 3,407.8 $ 1,757.2 $ 2,173.3 $ 1,205.5 The Company sells its products to distributors and original equipment manufacturers (OEMs) in a broad range of market segments, performs on-going credit evaluations of its customers and, as deemed necessary, may require collateral, primarily letters of credit. The Company's operations outside the U.S. consist of product assembly and final test facilities in Thailand, and sales and support centers and design centers in certain foreign countries. Domestic operations are responsible for the design, development and wafer fabrication of products, as well as the coordination of production planning and shipping to meet worldwide customer commitments. The Company's Thailand assembly and test facility is reimbursed in relation to value added with respect to assembly and test operations and other functions performed, and certain foreign sales offices receive compensation for sales within their territory. Accordingly, for financial statement purposes, it is not meaningful to segregate sales or operating profits for the assembly and test and foreign sales office operations. Identifiable long-lived assets (consisting of property, plant and equipment net of accumulated amortization) by geographic area are as follows (amounts in millions): March 31, 2018 2017 United States $ 393.3 $ 388.5 Thailand 215.5 178.0 Various other countries 159.1 116.8 Total long-lived assets $ 767.9 $ 683.3 Sales to unaffiliated customers located outside the U.S., primarily in Asia and Europe, aggregated approximately 85% of consolidated net sales for fiscal 2018 and 84% for each of fiscal 2017 and 2016 . Sales to customers in Europe represented approximately 24% of consolidated net sales for each of fiscal 2018 and 2017 , and approximately 22% of consolidated net sales for fiscal 2016 . Sales to customers in Asia represented approximately 58% of consolidated net sales for each of fiscal 2018 and 2017 , and approximately 59% of consolidated net sales for fiscal 2016 . Within Asia, sales into China, including Hong Kong, represented approximately 30% , 32% and 30% of consolidated net sales for fiscal 2018 , 2017 and 2016 , respectively. Sales into Taiwan represented approximately 11% , 9% and 12% of consolidated net sales for fiscal 2018 , 2017 and 2016 , respectively. Sales into any other individual foreign country did not exceed 10% of the Company's net sales for any of the three years presented. No single end customer or distributor accounted for 10% or more of the Company's net sales during fiscal 2018 , 2017 or 2016 .

Derivative Instruments

Derivative Instruments	12 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Instruments	Derivative Instruments Freestanding Derivative Forward Contracts The Company has international operations and is thus subject to foreign currency rate fluctuations. Approximately 99 % of the Company's sales are U.S. Dollar denominated. However, a significant amount of the Company's expenses and liabilities are denominated in foreign currencies and subject to foreign currency rate fluctuations. To help manage the risk of changes in foreign currency rates, the Company periodically enters into derivative contracts comprised of foreign currency forward contracts to hedge its asset and liability foreign currency exposure and a portion of its foreign currency operating expenses. Net gains due to foreign exchange rate fluctuations after the effects of hedging activity were $9.3 million , $1.0 million , and $0.7 million in fiscal 2018 , 2017 and 2016 . As of March 31, 2018 and 2017 , the Company had no foreign currency forward contracts outstanding. The Company recognized net realized gains on foreign currency forward contracts of $4.2 million in the year ended March 31, 2018 , compared to net losses of $2.3 million in 2017 and an immaterial amount in 2016 . Gains and losses from changes in the fair value of these foreign currency forward contracts and foreign currency exchange rate fluctuations are credited or charged to other income (expense). The Company does not apply hedge accounting to its foreign currency derivative instruments. Commodity Price Risk The Company is exposed to fluctuations in prices for energy that it consumes, particularly electricity and natural gas. The Company also enters into variable-priced contracts for some purchases of electricity and natural gas, on an index basis. The Company seeks, or may seek, to partially mitigate these exposures through fixed-price contracts. These contracts meet the characteristics of derivative instruments, but generally qualify for the "normal purchases or normal sales" exception under authoritative guidance and require no mark-to-market adjustment. Fair Value Hedges For derivative instruments that are designated and qualify as fair value hedges, the gain or loss on the derivatives as well as the offsetting gain or loss on the hedged item attributable to the hedged risk are recognized in earnings. Interest rate derivative instruments designated as fair value hedges are designed to manage the exposure to interest rate movements and to reduce borrowing costs by converting fixed-rate debt into floating-rate debt. Under these agreements, the Company agrees to exchange, at specified intervals, the difference between the fixed and floating interest amounts calculated by reference to an agreed-upon notional principal amount. In March 2015, the Company entered into ten -year fixed-to-floating interest rate swap agreements designated as fair value hedges of the changes in fair value of a portion of the Company's fixed-rate 1.625% 2015 Senior Debt due to changes in the LIBOR swap rate, the designated benchmark interest rate. The Company pays variable interest equal to the three-month LIBOR minus 53.6 basis points and it receives a fixed interest rate of 1.625% . The notional amount of these contracts outstanding at March 31, 2015 was $ 431.3 million , representing 25% of the principal amount of the 2015 Senior Debt. In February 2016, the Company terminated its interest rate swap agreements. Upon termination, the contracts were in an asset position, resulting in cash receipts of approximately $ 25.7 million , which included $3.7 million of accrued interest. The gain from terminating the interest rate swap agreements increased the outstanding balance of the 2015 Senior Debt and is being amortized as a reduction of interest expense over the remaining life of the debt. The cash flows from the termination of these interest rate swap agreements have been reported as operating activities in the consolidated statements of cash flows. The following table summarizes the location and amount of the gain or loss on the hedged item attributable to the changes in the LIBOR swap rate and the offsetting gain or loss on the related interest rate swap agreements for the year ended March 31, 2016. The difference represents hedge ineffectiveness (amounts in millions): Year ended March 31, 2016 Income Statement Classification Gain (Loss) on 2015 Senior Debt Gain (Loss) on Interest Rate Swap Other (loss) income, net $ (18.1 ) $ 16.3

Net Income Per Common Share Fro

Net Income Per Common Share From Continuing Operations Attributable to Microchip Technology Stockholders	12 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Net Income Per Common Share From Continuing Operations Attributable to Microchip Technology Stockholders	Net Income Per Common Share From Continuing Operations Attributable to Microchip Technology Stockholders The following table sets forth the computation of basic and diluted net income per common share from continuing operations attributable to Microchip stockholders (in millions, except per share amounts): Year Ended March 31, 2018 2017 2016 Net income from continuing operations attributable to Microchip Technology Stockholders $ 255.4 $ 170.6 $ 324.1 Weighted average common shares outstanding 232.9 217.2 203.4 Dilutive effect of stock options and RSUs 4.4 4.4 3.3 Dilutive effect of 2007 Junior Debt 1.3 12.7 10.7 Dilutive effect of 2015 Senior Debt 10.3 0.5 — Dilutive effect of 2017 Senior Debt — — — Dilutive effect of 2017 Junior Debt — — — Weighted average common and potential common shares outstanding 248.9 234.8 217.4 Basic net income per common share from continuing operations attributable to Microchip stockholders $ 1.10 $ 0.79 $ 1.59 Diluted net income per common share from continuing operations attributable to Microchip stockholders $ 1.03 $ 0.73 $ 1.49 The Company computed basic net income per common share from continuing operations attributable to its stockholders using net income from continuing operations available to common stockholders and the weighted average number of common shares outstanding during the period. The Company computed diluted net income per common share from continuing operations attributable to its stockholders using net income from continuing operations available to common stockholders and the weighted average number of common shares outstanding plus potentially dilutive common shares outstanding during the period. Potentially dilutive common shares from employee equity incentive plans are determined by applying the treasury stock method to the assumed exercise of outstanding stock options and the assumed vesting of outstanding RSUs. Weighted average common shares exclude the effect of option shares which are not dilutive. There were no anti-dilutive option shares for the years ended March 31, 2018 and March 31, 2017 . For the year ended March 31, 2016 , the number of option shares that were antidilutive was 0.3 million . Diluted net income per common share from continuing operations attributable to stockholders for fiscal 2018 , 2017 , and 2016 includes 1.3 million , 12.7 million and 10.7 million shares, respectively, issuable upon the exchange of the Company's 2007 Junior Debt. The Company's 2007 Junior Debt was fully settled as of December 31, 2017 (see Note 11 for details on the settlement of convertible debt), with the Company issuing an aggregate of 3.7 million shares in the settlement of $143.8 million principal amount in fiscal 2018 and an aggregate of 12.0 million shares in the settlement of $431.3 million principal amount in fiscal 2017. The shares that were issued are included in the weighted average dilutive common shares outstanding through the date of the issuance and were reflected in the weighted average common shares outstanding thereafter. Diluted net income per common share from continuing operations attributable to stockholders for fiscal 2018 and 2017 includes 10.3 million shares and 0.5 million shares, respectively, issuable upon the exchange of the Company's 2015 Senior Debt. The convertible debt has no impact on diluted net income per common share unless the average price of the Company's common stock exceeds the conversion price because the principal amount of the debentures will be settled in cash upon conversion. Prior to conversion, the Company will include, in the diluted net income per common share calculation, the effect of the additional shares that may be issued when the Company's common stock price exceeds the conversion price using the treasury stock method. The following is the weighted average conversion price per share used in calculating the dilutive effect (See Note 11 for details on the convertible debt): Year Ended March 31, 2018 2017 2016 2007 Junior Debt (1) $ 23.59 $ 24.01 $ 24.73 2015 Senior Debt $ 63.94 $ 65.21 $ 67.19 2017 Senior Debt $ 99.71 $ 100.58 $ — 2017 Junior Debt $ 97.96 $ 98.81 $ — (1) No longer outstanding as of December 31, 2018.

Quarterly Results (Unaudited)

Quarterly Results (Unaudited)	12 Months Ended
	Mar. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Quarterly Results (Unaudited)	Quarterly Results (Unaudited) The following table presents the Company's selected unaudited quarterly operating results for the eight quarters ended March 31, 2018 . The Company believes that all adjustments of a normal recurring nature have been made to present fairly the related quarterly results (in millions, except per share amounts. Amounts may not add to the total due to rounding): Fiscal 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total Net sales $ 972.1 $ 1,012.1 $ 994.2 $ 1,002.3 $ 3,980.8 Gross profit 584.4 614.1 607.1 615.1 2,420.7 Operating income 221.6 225.4 245.2 244.1 936.3 Net income (loss) attributable to Microchip Technology 170.6 189.2 (251.1 ) 146.7 255.4 Diluted net income (loss) per common share attributable to Microchip Technology stockholders 0.70 0.77 (1.07 ) 0.58 1.03 Fiscal 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total Net sales $ 799.4 $ 871.4 $ 834.4 $ 902.7 $ 3,407.8 Gross profit 348.5 410.6 465.3 532.8 1,757.2 Operating income (loss) (59.1 ) 62.8 118.1 154.1 275.8 Net income (loss) attributable to Microchip Technology (113.4 ) 33.9 107.2 136.9 164.6 Diluted net income (loss) per common share attributable to Microchip Technology stockholders (0.53 ) 0.14 0.46 0.57 0.71 Refer to Note 10, Income Taxes, for an explanation of the one-time transition tax recognized in the third quarter of fiscal 2018. Refer to Note 3, Special Charges and Other, Net, for an explanation of the special charges included in operating income in fiscal 2018 and fiscal 2017 . Refer to Note 11, Debt and Credit Facility, for an explanation of the loss on settlement of convertible debt included in net income (loss) of $13.8 million and $2.2 million for the first quarter and third quarter of fiscal 2018, respectively, and $43.9 million during the fourth quarter of fiscal 2017 . Refer to Note 4, Investments, for an explanation of the impairment recognized on available-for-sale securities in the fourth quarter of fiscal 2018. No material net realized gains or losses occurred in fiscal 2017.

Supplemental Financial Informat

Supplemental Financial Information	12 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]
Supplemental Financial Information	Supplemental Financial Information Cash paid for income taxes amounted to $ 25.9 million , $ 48.4 million and $ 25.4 million during fiscal 2018 , 2017 and 2016 , respectively. Cash paid for interest on borrowings amounted to $ 85.3 million in fiscal 2018 , $ 82.5 million in fiscal 2017 and $ 52.9 million in fiscal 2016 . A summary of additions and deductions related to the valuation allowance for deferred tax asset accounts for the years ended March 31, 2018 , 2017 and 2016 follows (amounts in millions): Balance at Beginning of Year Additions Charged to Costs and Expenses Additions Charged to Other Accounts Deductions Balance at End of Year Valuation allowance for deferred tax assets: Fiscal 2018 $ 210.1 $ 36.2 $ — $ (41.8 ) $ 204.5 Fiscal 2017 161.8 15.2 37.6 (4.5 ) 210.1 Fiscal 2016 116.5 5.5 47.8 (8.0 ) 161.8 A summary of additions and deductions related to the allowance for doubtful accounts for the years ended March 31, 2018 , 2017 and 2016 follows (amounts in millions): Balance at Beginning of Year Additions Charged to Costs and Expenses Deductions (1) Balance at End of Year Allowance for doubtful accounts: Fiscal 2018 $ 2.1 $ 0.2 $ (0.1 ) $ 2.2 Fiscal 2017 $ 2.5 $ 0.2 $ (0.6 ) $ 2.1 Fiscal 2016 $ 2.6 $ 0.1 $ (0.2 ) $ 2.5 (1) Deductions represent uncollectible accounts written off, net of recoveries. Accumulated Other Comprehensive Income The following tables present the changes in the components of accumulated other comprehensive income (AOCI) for the years ended March 31, 2018 and March 31, 2017 : Unrealized Holding Gains (Losses) Available-for-sale Securities Minimum Pension Liability Foreign Currency Total Balance at March 31, 2017 $ 0.3 $ (5.3 ) $ (9.4 ) $ (14.4 ) Other comprehensive loss before reclassifications (13.6 ) (5.6 ) — (19.2 ) Amounts reclassified from accumulated other comprehensive income (loss) 15.2 0.8 — 16.0 Net other comprehensive income (loss) 1.6 (4.8 ) — (3.2 ) Balance at March 31, 2018 $ 1.9 $ (10.1 ) $ (9.4 ) $ (17.6 ) Unrealized Holding Gains (Losses) Available-for-sale Securities Minimum Pension Liability Foreign Currency Total Balance at March 31, 2016 $ 0.3 $ — $ (3.7 ) $ (3.4 ) Other comprehensive loss before reclassifications (1.5 ) (5.3 ) (5.7 ) (12.5 ) Amounts reclassified from accumulated other comprehensive income (loss) 1.5 — — 1.5 Net other comprehensive loss — (5.3 ) (5.7 ) (11.0 ) Balance at March 31, 2017 $ 0.3 $ (5.3 ) $ (9.4 ) $ (14.4 ) The table below details where reclassifications of realized transactions out of AOCI are recorded on the consolidated statements of income (amounts in millions). Year ended March 31, Description of AOCI Component 2018 2017 2016 Related Statement of Income Line Unrealized (losses) gains on available-for-sale securities $ (15.5 ) $ (1.5 ) $ 10.9 Other income, net Amortization of actuarial loss (0.8 ) — — Other income, net Taxes 0.3 — — Provision for income taxes Reclassification of realized transactions, net of taxes $ (16.0 ) $ (1.5 ) $ 10.9 Net Income

Dividends

Dividends	12 Months Ended
	Mar. 31, 2018
Dividends [Abstract]
Dividends	Dividends On October 28, 2002, the Company announced that its Board of Directors had approved and instituted a quarterly cash dividend on its common stock. The Company has continued to pay quarterly dividends and has increased the amount of such dividends on a regular basis. Cash dividends paid per share were $ 1.449 , $ 1.441 and $ 1.433 during fiscal 2018 , 2017 and 2016 , respectively. Total dividend payments amounted to $337.5 million , $ 315.4 million and $ 291.1 million during fiscal 2018 , 2017 and 2016 , respectively.

Subsequent Events

Subsequent Events	12 Months Ended
	Mar. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events Merger Agreement On March 1, 2018 , the Company entered into a definitive agreement to acquire Microsemi for $68.78 per share in cash. The acquisition price represents a total equity value of approximately $8.35 billion , and a total enterprise value of about $10.15 billion , after accounting for Microsemi's cash and investments, net of debt, on its balance sheet at December 2017 . Microsemi offers a comprehensive portfolio of semiconductor and system solutions for aerospace & defense, communications, data center and industrial markets. Microsemi is headquartered in Aliso Viejo, California, and has approximately 4,800 employees globally. The Board of Directors of each of Microchip and Microsemi have unanimously approved the Merger and the Merger Agreement. The completion of the merger is subject to obtaining regulatory approvals and certain other closing conditions, including approval by Microsemi stockholders. Assuming the timely receipt of the regulatory approvals and the satisfaction of the other closing conditions, Microchip anticipates that the merger will be completed in late May/early June 2018. The Merger Agreement contains customary representations, warranties and covenants of Microchip, Microsemi, and Merger Sub, including among others, (i) covenants by Microsemi concerning the conduct of its business in the ordinary course consistent with past practice during the interim period between the execution of the Merger Agreement and the consummation of the Merger, (ii) a covenant by Microsemi that, subject to certain exceptions, the Board of Directors of Microsemi will recommend to its stockholders adoption of the Merger Agreement, and (iii) a covenant that Microsemi will not solicit, initiate or knowingly encourage, facilitate or induce the making, submission or announcement of an Acquisition Proposal (as defined in the Merger Agreement) or the making of any inquiry, offer or proposal that would reasonably be expected to lead to an Acquisition Proposal. The Merger Agreement contains certain termination rights for both Microchip and Microsemi and further provides that upon termination of the Merger Agreement under specified circumstances (including termination by Microsemi to accept a superior proposal), Microsemi may be required to pay Microchip a termination fee of $290 million . The Merger Agreement also contains a provision requiring Microsemi to reimburse Microchip for up to $35 million of its expenses incurred in connection with the Merger if the approval of the Merger by the holders of a majority of the outstanding shares of Microsemi Common Stock is not obtained at a meeting called for that purpose. Financing of the Merger On March 1, 2018, in connection with its execution of the Merger Agreement, Microchip entered into a Commitment Letter with JPMorgan, pursuant to which JPMorgan committed to provide to Microchip, among other things, (1) a senior secured term loan facility in an aggregate principal amount of up to $5.0 billion and (2) a 364 -day secured bridge facility in an aggregate principal amount of up to $625.0 million , in each case, subject to the execution of definitive documentation and customary closing conditions. JPMorgan’s commitments in respect of the Term Loan Facility will be reduced in the event Microchip raises certain other debt or equity financing. Microchip estimates that the total amount of funds necessary to complete the acquisition of Microsemi and the other transactions contemplated by the Merger Agreement will be approximately $10.15 billion to: • make payments to Microsemi’s stockholders of the amounts due to them under the Merger Agreement; • make payments in respect of Microsemi’s outstanding equity-based awards pursuant to the Merger Agreement; • refinance or otherwise discharge outstanding indebtedness of Microsemi that is required to be refinanced or discharged pursuant to the Merger Agreement, including, without limitation, the termination of commitments and the refinancing of indebtedness under Microsemi’s existing credit agreement, and the optional redemption of Microsemi’s 9.125% Senior Notes due 2023; and • pay related fees, expenses and taxes in connection with the foregoing and consummation of the Merger. Microchip expects to finance the foregoing through a combination of: • the incurrence of loans under a new term loan facility and other debt financing; • revolving loans under Microchip’s amended and restated credit agreement; and • Microchip’s and Microsemi’s cash and cash equivalents on hand at closing. Credit Facility Amendment On May 18, 2018 (the “ Effective Date ”), the Company entered into an amended and restated credit agreement (the “ Amended and Restated Credit Agreement ”) pursuant to which its existing Amended and Restated Credit Agreement, dated as of June 27, 2013, as amended and restated as of February 4, 2015, and as further amended, restated amended and restated, supplemented or otherwise modified from time to time prior to the Effective Date, by and among the Company, the lenders from time to time party thereto and JPMorgan Chase Bank, N.A., as administrative agent, was amended and restated in its entirety. The Amended and Restated Credit Agreement provides for a revolving loan facility (the “ Revolving Loan Facility ”) in an aggregate principal amount of approximately $3.8 billion , with a $250.0 million foreign currency sublimit, a $50.0 million letter of credit sublimit and a $25.0 million swingline loan sublimit. The Revolving Loan Facility consists of approximately $244.3 million of revolving loan commitments (the “ 2020 Revolving Loans ”) that terminate on February 4, 2020 (the “ 2020 Maturity Date ”) and approximately $3.6 billion of revolving loan commitments (the “ 2023 Revolving Loans ” and, together with the 2020 Revolving Loans, the “ Revolving Loans ”) that terminate on May 18, 2023 (the “ 2023 Maturity Date ”). The Revolving Loans bear interest, at the Company’s option, at (a) in the case of 2020 Revolving Loans, the base rate plus a spread of 0.25% to 1.25% or an adjusted LIBOR rate plus a spread of 1.25% to 2.25% , and (b) in the case of 2023 Revolving Loans, the base rate plus a spread of 0.00% to 1.00% or an adjusted LIBOR rate plus a spread of 1.00% to 2.00% , in each case, with such spread being determined based on the consolidated senior leverage ratio for the preceding four fiscal quarter period. The Amended and Restated Credit Agreement contains customary representations and warranties, affirmative covenants, negative covenants, financial covenants and events of default. The Amended and Restated Credit Agreement is guaranteed by certain of our subsidiaries and secured by certain assets of ours and the guarantor subsidiaries. Building Purchase During the quarter ended March 31, 2018, the Company entered into an agreement to purchase one of the buildings it leased in San Jose, California for a purchase price of $40.8 million with a closing date of April 4, 2018.

Significant Accounting Polici34

Significant Accounting Policies (Policies)	12 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The Company prepares its consolidated financial statements in accordance with generally accepted accounting principles in the United States of America (US GAAP). The consolidated financial statements include the accounts of Microchip and its majority-owned and controlled subsidiaries. All of the Company's subsidiaries are included in the consolidated financial statements. All significant intercompany accounts and transactions have been eliminated in consolidation.
Revenue Recognition	Revenue Recognition (prior to the adoption of ASC 606) The Company recognizes revenue when the earnings process is complete, as evidenced by an agreement with the customer, transfer of title has occurred, the pricing is fixed or determinable and collectability is reasonably assured. The Company recognizes revenue from product sales to original equipment manufacturers (OEMs) upon shipment and records reserves for estimated customer returns. Distributors worldwide generally have broad price protection and product return rights which prevent the sales pricing from being fixed or determinable at the time of the Company's shipment to the distributors. Therefore, revenue recognition is deferred until the pricing uncertainty is resolved, which generally occurs when the distributor sells the product to their customer. At the time of shipment to these distributors, the Company records a trade receivable for the selling price as there is a legally enforceable right to payment, relieves inventory for the carrying value of goods shipped since legal title has passed to the distributor, and records the gross margin in deferred income on shipments to distributors on its consolidated balance sheets. In connection with its acquisitions of Atmel and Micrel, the Company acquired certain distributor relationships where revenue was recognized upon shipment to the distributors based on certain contractual terms or prevailing business practices that resulted in the price being fixed and determinable at such time. Following an acquisition, the Company undertakes efforts to align the contract terms and business practices of the acquired entity with its own. Once these efforts are complete, the related revenue recognition is changed. With respect to such distributor relationships acquired in the Atmel acquisition, as of October 1, 2016, these business practices were conformed to those of the Company’s other distributors, which beginning in October 2016 resulted in the deferral of revenue recognition until the distributor sells the product to their customers. With respect to such distributor relationships acquired in the Micrel acquisition, in the December 2015 quarter, these distributor contracts were changed to be consistent with those of the Company’s other distributors which resulted in the deferral of revenue recognition under such contracts until the distributor sells the product to their customers. Deferred income on shipments to distributors effectively represents gross margin on the sale to the distributor at the initial shipment date; however, the amount of gross margin recognized by the Company in future periods will be less than the deferred margin as a result of credits granted to distributors on specifically identified products and customers to allow the distributors to earn a competitive gross margin on the sale of the Company's products to their end customers and price protection concessions related to market pricing conditions. The Company sells the majority of the items in its product catalog to its distributors worldwide at a uniform list price. However, distributors resell the Company's products to end customers at a broad range of individually negotiated price points. The majority of the Company's distributors' resales require a reduction from the original list price paid. Often, under these circumstances, the Company remits back to the distributor a portion of their original purchase price after the resale transaction is completed in the form of a credit against the distributors' outstanding accounts receivable balance. The credits are on a per unit basis and are not given to the distributor until they provide information regarding the sale to their end customer. The price reductions vary significantly based on the customer, product, quantity ordered, geographic location and other factors and discounts to a price less than the Company's cost have historically been rare. The effect of granting these credits establishes the net selling price from the Company to its distributors for the product and results in the net revenue recognized by the Company when the product is sold by the distributors to their end customers. Thus, a portion of the "deferred income on shipments to distributors" balance represents the amount of distributors' original purchase price that will be credited back to the distributors in the future. The Company does not reduce deferred income on shipments to distributors or accounts receivable by anticipated future price concessions; rather, price concessions are recorded against deferred income on shipments to distributors when incurred, which is generally at the time the distributor sells the product. At March 31, 2018 , the Company had approximately $479.6 million of deferred revenue and $145.8 million in deferred cost of sales recognized as $ 333.8 million of deferred income on shipments to distributors. At March 31, 2017 , the Company had approximately $418.0 million of deferred revenue and $125.2 million in deferred cost of sales recognized as $ 292.8 million of deferred income on shipments to distributors. The deferred income on shipments to distributors that will ultimately be recognized in the Company's income statement will be lower than the amount reflected on the balance sheet due to price credits to be granted to the distributors when the product is sold to their customers. These price credits historically have resulted in the deferred income approximating the overall gross margins that the Company recognizes in the distribution channel of its business. The Company reduces product pricing through price protection based on market conditions, competitive considerations and other factors. Price protection is granted to distributors on the inventory they have on hand at the date the price protection is offered. When the Company reduces the price of its products, it allows the distributor to claim a credit against its outstanding accounts receivable balances based on the new price of the inventory it has on hand as of the date of the price reduction. There is no immediate revenue impact from the price protection, as it is reflected as a reduction of the deferred income on shipments to distributors' balance. Products returned by distributors and subsequently scrapped have historically been immaterial to the Company's consolidated results of operations. The Company routinely evaluates the risk of impairment of the deferred cost of sales component of the deferred income on shipments to distributors' account. Because of the historically immaterial amounts of inventory that have been scrapped, and historically rare instances where discounts given to a distributor result in a price less than the Company's cost, the Company believes the deferred costs have a low risk of material impairment. Shipping charges billed to customers are included in net sales, and the related shipping costs are included in cost of sales. The Company collects and remits certain sales-related taxes on a portion of its sales of inventory and reports such amounts under the net method in its consolidated statements of income. For licenses or other technology arrangements without an upgrade period, non-royalty revenue from the license is recognized upon delivery of the technology if the fee is fixed or determinable and collection of the fee is reasonably assured. Royalties are recognized when reported to the Company, which generally coincides with the receipt of payment. In certain limited circumstances, the Company enters into license and other arrangements for technologies that the Company is continuing to enhance and refine or under which it is obligated to provide unspecified enhancements. Under these arrangements, non-royalty revenue is recognized over the lesser of (1) the estimated period that the Company has historically enhanced and developed refinements to the specific technology, typically one to three years (the "upgrade period"), and (2) the remaining portion of the upgrade period after the date of delivery of all specified technology and documentation, provided that the fee is fixed or determinable and collection of the fee is reasonably assured. Royalties received during the upgrade period are recognized as revenue based on an amortization calculation of the elapsed portion of the upgrade period compared to the entire estimated upgrade period. Royalties received after the upgrade period has elapsed are recognized when reported to the Company, which generally coincides with the receipt of payment. Changes to Revenue Recognition Resulting From the Adoption of ASC 606 In May 2014, the Financial Accounting Standards Board (FASB) issued ASU 2014-09 -Revenue from Contracts with Customers (Topic 606) , which superseded nearly all existing revenue recognition guidance under generally accepted accounting principles in the United States of America (US GAAP). In August 2015, the FASB issued ASU 2015-14- Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delayed the effective date of the new standard by one year to December 15, 2017, for annual and interim reporting periods beginning after that date. In accordance with the delay, the new standard was effective for the Company beginning April 1, 2018. The standard's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The new standard allows for the amendment to be applied either retrospectively to each prior reporting period presented or retrospectively as a cumulative-effect adjustment as of the date of adoption. In March 2016, the FASB issued ASU 2016-08- Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net) , which clarifies the implementation guidance on principal versus agent considerations. In April 2016, the FASB issued ASU 2016-10- Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , which clarifies the implementation guidance on identifying performance obligations. In May 2016, the FASB issued ASU 2016-12- Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients , which addresses implementation issues that were raised by stakeholders and discussed by the Revenue Recognition Transition Resource Group. For a discussion of the financial statement impact related to the adoption of these standards, see “Recently Issued Accounting Pronouncements Not Yet Adopted”.
Product Warranty	Product Warranty The Company typically warrants its products against defects in materials and workmanship and non-conformance to specifications for 12 to 24 months. The majority of the Company's product warranty claims are settled through the return of the defective product and the shipment of replacement product. Warranty returns are included within the Company's allowance for returns, which is based on historical return rates. Actual future returns could differ from the allowance established. In addition, the Company accrues a liability for specific warranty costs expected to be settled other than through product return and replacement, if a loss is probable and can be reasonably estimated. Product warranty expenses were immaterial for the fiscal years ended March 31, 2018 , 2017 , and 2016 .
Advertising Costs	Advertising Costs The Company expenses all advertising costs as incurred.
Research and Development	Research and Development Research and development costs are expensed as incurred. Assets purchased to support the Company's ongoing research and development activities are capitalized when related to products which have achieved technological feasibility or that have alternative future uses and are amortized over their estimated useful lives. Renewals or extensions of these assets are expensed as incurred. Research and development expenses include expenditures for labor, share-based payments, depreciation, masks, prototype wafers, and expenses for development of process technologies, new packages, and software to support new products and design environments.
Restructuring Charges	Restructuring Charges The Company recognizes a liability measured at fair value for restructuring costs when the liability is incurred. Restructuring charges are included within special charges and other, net in the consolidated statements of income and are primarily comprised of employee separation costs, asset impairments, contract exit costs and costs of facility consolidation and closure, including the related gains or losses associated with the sale of owned facilities. Employee separation costs includes one-time termination benefits that are recognized as a liability at estimated fair value, at the time of communication to employees, unless future service is required, in which case the costs are recognized ratably over the future service period. Ongoing termination benefits are recognized as a liability at estimated fair value when the amount of such benefits are probable and reasonably estimable. Contract exit costs includes contract termination fees and future contractual commitments for lease payments. A liability for contract exit costs is recognized in the period in which the Company terminates the contract or on the cease-use date for leased facilities.
Foreign Currency Translation	Foreign Currency Translation The Company's foreign subsidiaries are considered to be extensions of the U.S. company and any translation gains and losses related to these subsidiaries are included in other income (expense) in the consolidated statements of income. As the U.S. dollar is utilized as the functional currency, gains and losses resulting from foreign currency transactions (transactions denominated in a currency other than the subsidiaries' functional currency) are also included in income. For a portion of fiscal 2017, certain foreign subsidiaries acquired as part of the Company's acquisition activities had the local currency as the functional currency. Once these entities were integrated into the Company's legal structure and intercompany agreements were executed, the U.S. dollar became the functional currency for such entities.
Income Taxes	Income Taxes As part of the process of preparing its consolidated financial statements, the Company is required to estimate its income taxes in each of the jurisdictions in which it operates. This process involves estimating its actual current tax exposure together with assessing temporary differences resulting from differing treatment of items for tax and accounting purposes. These differences result in deferred tax assets and liabilities, which are included within the Company's consolidated balance sheets. The Company must then assess the likelihood that its deferred tax assets will be recovered from future taxable income within the relevant jurisdiction and to the extent the Company believes that recovery is not likely, it must establish a valuation allowance. The Company provided valuation allowances for certain of its deferred tax assets, including state net operating loss carryforwards and state tax credits, where it is more likely than not that some portion, or all of such assets, will not be realized. Due to the Tax Cuts and Jobs Act (the "Act"), the Company released its valuation allowance on foreign tax credits during the period ending March 31, 2018, which was provisional. The Company is still evaluating how the Act impacts its valuation allowance on state net operating loss carryforwards and state tax credits, and it may report an adjustment to the valuation allowances under Staff Accounting Bulletin ("SAB") 118 in subsequent quarters. Should the Company determine that it would not be able to realize all or part of its net deferred tax asset in the future, an adjustment to the deferred tax asset would be charged to income in the period such determination was made. Various taxing authorities in the U.S. and other countries in which the Company does business scrutinize the tax structures employed by businesses. Companies of a similar size and complexity as the Company are regularly audited by the taxing authorities in the jurisdictions in which they conduct significant operations. During the year ended March 31, 2018, various foreign jurisdictions finalized their audits. The close of these audits did not have an adverse impact on our financial statements. The Company is currently being audited by the tax authorities in the United States and various foreign jurisdictions. At this time, the Company does not know what the outcome of these audits will be. We record benefits for uncertain tax positions based on an assessment of whether it is more likely than not that the tax positions will be sustained based on their technical merits under currently enacted law. If this threshold is not met, no tax benefit of the uncertain tax position is recognized. If the threshold is met, we recognize the largest amount of the tax benefit that is more than 50% likely to be realized upon ultimate settlement. The accounting model as defined in Accounting Standards Codification ("ASC") 740 related to the valuation of uncertain tax positions requires the Company to presume that the tax position will be examined by the relevant taxing authority that has full knowledge of all relevant information and that each tax position will be evaluated without consideration of the possibility of offset or aggregation with other positions. The recognition requirement for the liability exists even if the Company believes the possibility of examination by a taxing authority or discovery of the related risk matters is remote or where it has a long history of the taxing authority not performing an exam or overlooking an issue. The Company will record an adjustment to a previously recorded position if new information or facts related to the position are identified in a subsequent period. All adjustments to the positions are recorded through the income statement. Generally, adjustments will be recorded in periods subsequent to the initial recognition if the taxing authority has completed an audit of the period or if the statute of limitation expires. Due to the inherent uncertainty in the estimation process and in consideration of the criteria of the accounting model, amounts recognized in the financial statements in periods subsequent to the initial recognition may significantly differ from the estimated exposure of the position under the accounting model. On December 22, 2017, the Act was enacted into law. The Act provides for numerous significant tax law changes and modifications including the reduction of the U.S. federal corporate income tax rate from 35.0% to 21.0%, the requirement for companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and the creation of new taxes on certain foreign-sourced earnings. As a fiscal year-end taxpayer, certain provisions of the Act began to impact the Company in the third quarter of fiscal 2018, while other provisions will impact the Company beginning in fiscal 2019. The corporate tax rate reduction is effective as of January 1, 2018. Since the Company has a fiscal year rather than a calendar year, it is subject to rules relating to transitional tax rates. As a result, the Company's fiscal 2018 federal statutory rate will be a blended rate of 31.5% . In addition to the impacts of tax reform on fiscal 2018, the Act establishes new tax laws that will be effective for fiscal 2019, including, but not limited to, (1) a new provision designed to tax low-taxed income of foreign subsidiaries, which allows for the possibility of using foreign tax credits ("FTCs") and a deduction of up to 50% to offset the income tax liability (subject to some limitations); (2) limitations on the deductibility of certain executive compensation; (3) limitations on the deductibility of interest expense; and (4) limitations on the use of FTCs to reduce the U.S. income tax liability. While each of these provisions is expected to have an impact on the Company's tax expense for fiscal 2019 and future periods, the Company expects the tax on low-taxed income of foreign subsidiaries to have the most significant, adverse impact on its results. Due to the complexity of the new tax on low-taxed income of foreign subsidiaries, the Company is continuing to evaluate this provision of the Act and the application of ASC 740. Based on recent FASB deliberations, it appears the Company will be allowed to make an accounting policy choice of either (1) treating taxes due on future U.S. inclusions in taxable income as a current-period expense when incurred or (2) factoring such amounts into its measurement of deferred taxes. The Company's selection of an accounting policy will depend, in part, on analyzing its facts to determine what the impact is expected to be under each method.
Cash and Cash Equivalents	Cash and Cash Equivalents All highly liquid investments, including marketable securities with an original maturity to the Company of three months or less when acquired are considered to be cash equivalents.
Available-for-Sale Investments	Available-for-Sale Investments The Company classifies its investments in debt and marketable equity securities as available-for-sale based upon management's intent with regard to the investments and the nature of the underlying securities. The Company's available-for-sale investments consist of government agency bonds, municipal bonds, corporate bonds and marketable equity securities. The Company's investments are carried at fair value with unrealized gains and losses reported in stockholders' equity unless losses are considered to be other than temporary impairments in which case the losses are recognized through the statement of income. Premiums and discounts are amortized or accreted over the life of the related available-for-sale security. Dividend and interest income are recognized when earned. The cost of available-for-sale debt securities sold is calculated using the first-in, first-out (FIFO) basis at the individual security level for sales from multiple lots. For sales of marketable equity securities, the Company uses an average cost basis at the individual security level. The Company includes within short-term investments its income yielding available-for-sale securities that can be readily converted to cash and includes within long-term investments those income yielding available-for-sale securities with maturities of over one year that have unrealized losses attributable to them or those that cannot be readily liquidated. As discussed in Note 4, if the Company anticipates that investments will be liquidated in anticipation of a pending merger, any unrealized losses are recognized as other-than-temporary impairments. If the anticipated liquidity event is within 12 months, the securities are classified as short-term investments. In the normal course of business, the Company intends and has the ability to hold its long-term investments with temporary impairments until such time as these assets are no longer impaired. As of March 31, 2018, such recovery of unrealized losses is not expected to occur within the next year as the investments will be sold.
Derivative Instruments	Derivative Instruments Derivative instruments are required to be recorded at fair value as either assets or liabilities in the Company's consolidated balance sheet. The Company's accounting policies for derivative instruments depends on whether the instrument has been designated and qualifies as part of a hedging relationship and further, on the type of hedging relationship. The Company does not apply hedge accounting to foreign currency forward contracts. Gains and losses associated with currency rate changes on forward contracts are recorded currently in income. These gains and losses have been immaterial to the Company's financial statements. The Company is exposed to fluctuations in prices for energy that it consumes, particularly electricity and natural gas. The Company also enters into variable-priced contracts for some purchases of electricity and natural gas, on an index basis. The Company seeks, or may seek, to partially mitigate these exposures through fixed-price contracts. These contracts meet the characteristics of derivative instruments, but generally qualify for the “normal purchases or normal sales” exception under authoritative guidance and require no mark-to-market adjustment. Interest rate derivative instruments designated as fair value hedges are designed to manage the exposure to interest rate movements and to reduce borrowing costs by converting fixed-rate debt into floating-rate debt. Under these agreements, the Company agrees to exchange, at specified intervals, the difference between the fixed and floating interest amounts calculated by reference to an agreed-upon notional principal amount. For derivative instruments that are designated and qualify as fair value hedges, the gain or loss on the derivatives as well as the offsetting gain or loss on the hedged item attributable to the hedged risk are recognized in earnings. The Company evaluates hedge effectiveness at inception and on an ongoing basis. If a derivative is no longer expected to be highly effective, hedge accounting is discontinued.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts The Company maintains an allowance for doubtful accounts for probable losses on uncollectible accounts receivable resulting from the inability of its customers to make required payments, which is included in bad debt expense. The Company determines the adequacy of this allowance by routinely analyzing the composition of accounts receivable and evaluating customer creditworthiness, credit history, facts and circumstances specific to outstanding balances and current economic conditions.
Inventories	Inventories Inventories are valued at the lower of cost or net realizable value using the first-in, first-out method. The Company writes down its inventory for estimated obsolescence or unmarketable inventory in an amount equal to the difference between the cost of inventory and the estimated net realizable value based upon assumptions about future demand and market conditions. If actual market conditions are less favorable than those projected by the Company, additional inventory write-downs may be required. Inventory impairment charges establish a new cost basis for inventory and charges are not subsequently reversed to income even if circumstances later suggest that increased carrying amounts are recoverable. In estimating reserves for obsolescence, the Company primarily evaluates estimates of demand over a 12-month period and provides reserves for inventory on hand in excess of the estimated 12-month demand. Estimates for projected 12-month demand are generally based on the average shipments of the prior three-month period, which are then annualized to adjust for any potential seasonality in the Company's business. The estimated 12-month demand is compared to the Company's most recently developed sales forecast to further reconcile the 12-month demand estimate. Management reviews and adjusts the estimates as appropriate based on specific situations. For example, demand can be adjusted up for new products for which historic sales are not representative of future demand. Alternatively, demand can be adjusted down to the extent any existing products are being replaced or discontinued. In periods where the Company's production levels are substantially below normal operating capacity, unabsorbed overhead production costs associated with the reduced production levels of the Company's manufacturing facilities are charged directly to cost of sales.
Property, Plant and Equipment	Property, Plant and Equipment Property, plant and equipment are stated at cost. Major renewals and improvements are capitalized, while maintenance and repairs are expensed when incurred. The Company's property and equipment accounting policies incorporate estimates, assumptions and judgments relative to the useful lives of its property and equipment. Depreciation is provided for assets placed in service on a straight-line basis over the estimated useful lives of the relative assets, which range from 10 to 40 years for buildings and building improvements and 3 to 7 years for machinery and equipment. The Company evaluates the carrying value of its property and equipment when events or changes in circumstances indicate that the carrying value of such assets may be impaired. Asset impairment evaluations are, by nature, highly subjective.
Senior and Junior Subordinated Convertible Debt	Senior and Junior Subordinated Convertible Debt The Company separately accounts for the liability and equity components of its senior and junior subordinated convertible debt in a manner that reflects its nonconvertible debt (unsecured debt) borrowing rate when interest cost is recognized. This results in a bifurcation of a component of the debt, classification of that component in equity and the accretion of the resulting discount on the debt to be recognized as part of interest expense in its consolidated statements of income. Lastly, the Company includes the dilutive effect of the shares of its common stock issuable upon conversion of the outstanding senior and junior subordinated convertible debt in its diluted income per share calculation regardless of whether the market price triggers or other contingent conversion features have been met. The Company applies the treasury stock method as it has the intent and ability to settle the principal amounts of the senior and junior subordinated convertible debentures in cash. This method results in incremental dilutive shares when the average market value of the Company's common stock for a reporting period exceeds the conversion prices per share and adjust as dividends are recorded in the future. Upon a de-recognition event such as a settlement or conversion, the Company estimates the fair value of the liability component and compares that to the carrying amount in order to calculate the appropriate amount of gain or loss. The remaining amounts paid or issued (in the case of non cash consideration in the form of shares of common stock) are recognized as a reduction of additional paid-in-capital. The fair value of the liability component is estimated using the current comparable borrowing rate for an otherwise identical non-convertible debt instrument.
Defined Benefit Pension Plans	Defined Benefit Pension Plans The Company maintains defined benefit pension plans, covering certain of its foreign employees. For financial reporting purposes, net periodic pension costs and pension obligations are determined based upon a number of actuarial assumptions, including discount rates for plan obligations, and assumed rates of compensation increases for employees participating in plans. These assumptions are based upon management's judgment and consultation with actuaries, considering all known trends and uncertainties.
Contingencies	Contingencies In the ordinary course of business, the Company is exposed to various liabilities as a result of contracts, product liability, customer claims and other matters. Additionally, the Company is involved in a limited number of legal actions, both as plaintiff and defendant. Consequently, the Company could incur uninsured liability in any of those actions. The Company also periodically receives notifications from various third parties alleging infringement of patents or other intellectual property rights, or from customers requesting reimbursement for various costs. With respect to pending legal actions to which the Company is a party and other claims, although the outcomes are generally not determinable, the Company believes that the ultimate resolution of these matters will not have a material adverse effect on its financial position, cash flows or results of operations. Litigation and disputes relating to the semiconductor industry are not uncommon, and the Company is, from time to time, subject to such litigation and disputes. As a result, no assurances can be given with respect to the extent or outcome of any such litigation or disputes in the future. The Company accrues for claims and contingencies when losses become probable and reasonably estimable. As of the end of each applicable reporting period, the Company reviews each of its matters and, where it is probable that a liability has been or will be incurred, it accrues for all probable and reasonably estimable losses. Where the Company can reasonably estimate a range of losses it may incur regarding such a matter, it records an accrual for the amount within the range that constitutes its best estimate. If the Company can reasonably estimate a range but no amount within the range appears to be a better estimate than any other, it uses the amount that is the low end of such range.
Business Combinations	Business Combinations All of the Company's business combinations are accounted for at fair value under the acquisition method of accounting. Under the acquisition method of accounting, (i) acquisition-related costs, except for those costs incurred to issue debt or equity securities, will be expensed in the period incurred; (ii) non-controlling interests will be valued at fair value at the acquisition date; (iii) in-process research and development will be recorded at fair value as an intangible asset at the acquisition date and amortized once the technology reaches technological feasibility; (iv) restructuring costs associated with a business combination will be expensed subsequent to the acquisition date; and (v) changes in deferred tax asset valuation allowances and income tax uncertainties after the acquisition date will be recognized through income tax expense or directly in contributed capital. The aggregate amount of consideration paid by the Company is allocated to net tangible assets and intangible assets based on their estimated fair values as of the acquisition date. The excess of the purchase price over the value of the net tangible assets and intangible assets is recorded to goodwill. The measurement of fair value of assets acquired and liabilities assumed requires significant judgment. The valuation of intangible assets, in particular, requires that the Company use valuation techniques such as the income approach. The income approach includes the use of a discounted cash flow model, which includes discounted cash flow scenarios and requires the following significant estimates: revenue, expenses, capital spending and other costs, and discount rates based on the respective risks of the cash flows.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets The Company's intangible assets include goodwill and other intangible assets. Goodwill is recorded when the purchase price paid for an acquisition exceeds the estimated fair value of the net identified tangible and intangible assets acquired. Other intangible assets include existing technologies, core and developed technology, in-process research and development, trademarks and trade names, distribution rights and customer-related intangibles. In-process research and development is capitalized until such time the related projects are completed or abandoned at which time the capitalized amounts will begin to be amortized or written off. Indefinite-lived intangible assets consist of goodwill and in-process research and development intangible assets that have not yet been placed in service. All other intangible assets are definite-lived intangible assets, including in-process research and development assets that have been placed in service, and are amortized over their respective estimated lives, ranging from 1 to 15 years. The Company is required to perform an impairment review of indefinite-lived intangible assets, including goodwill annually, and more frequently under certain circumstances. Indefinite-lived intangible assets are subjected to this annual impairment test during the fourth quarter of the Company's fiscal year. The Company engages primarily in the development, manufacture and sale of semiconductor products as well as technology licensing. As a result, the Company concluded there are two reporting units, semiconductor products and technology licensing. Under the qualitative indefinite-lived intangible asset impairment assessment standard, management evaluates whether it is more likely than not that the indefinite-lived intangible assets are impaired. If it is determined that it is more likely than not, the Company proceeds with the next step of the impairment test, which compares the fair value of the reporting unit or indefinite-lived intangible asset to its carrying value. If the Company determines through the impairment process that the indefinite-lived intangible asset has been impaired, the Company will record the impairment charge in its results of operation. Through March 31, 2018 , the Company has not had impaired goodwill. In the event that facts and circumstances indicate definite-lived intangible assets may be impaired, the Company evaluates the recoverability and estimated useful lives of such assets. If such indicators are present, recoverability is evaluated based on whether the sum of the estimated undiscounted cash flows attributable to the asset (group) in question is less than their carrying value. If less, the Company measures the fair value of the asset (group) and recognizes an impairment loss if the carrying amount of the assets exceeds their respective fair values.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company assesses whether indicators of impairment of long-lived assets are present. If such indicators are present, the Company determines whether the sum of the estimated undiscounted cash flows attributable to the assets in question is less than their carrying value. If less, the Company recognizes an impairment loss based on the excess of the carrying amount of the assets over their respective fair values. Fair value is determined by discounted future cash flows, appraisals or other methods. If the assets determined to be impaired are to be held and used, the Company recognizes an impairment loss through a charge to operating results to the extent the present value of anticipated net cash flows attributable to the asset are less than the asset's carrying value. The Company would depreciate the remaining value over the remaining estimated useful life of the asset.
Share-Based Compensation	Share-Based Compensation The Company has equity incentive plans under which non-qualified stock options and restricted stock units (RSUs) have been granted to employees and non-employee members of the Board of Directors. For the past several years the Company has adopted RSUs as its primary equity incentive compensation instrument for employees. The Company also has employee stock purchase plans for eligible employees. Share-based compensation cost is measured on the grant date based on the fair market value of the Company’s common stock discounted for expected future dividends and is recognized as expense on a straight-line basis over the requisite service periods. If there are any modifications or cancellations of the underlying unvested securities, the Company may be required to accelerate or increase any remaining unearned share-based compensation expense. Future share-based compensation expense and unearned share-based compensation will increase to the extent that the Company grants additional equity awards to employees or it assumes unvested equity awards in connection with acquisitions. During fiscal 2017, the Company elected to early adopt ASU 2016-09- Compensation - Stock Compensation, Improvements to Employee Share-Based Payment Accounting (Topic 718). See "Recently Adopted Accounting Pronouncements" for additional information on the new guidance.
Concentrations of Credit Risk	Concentrations of Credit Risk Financial instruments that potentially subject the Company to concentrations of credit risk consist primarily of investments in debt securities and trade receivables. Investments in debt securities with original maturities of greater than six months consist primarily of AAA and AA rated financial instruments and counterparties. The Company's investments are primarily in direct obligations of the U.S. government or its agencies, corporate bonds, and municipal bonds. Concentrations of credit risk with respect to accounts receivable are generally not significant due to the diversity of the Company's customers and geographic sales areas. The Company sells its products primarily to OEMs and distributors in the Americas, Europe and Asia. The Company performs ongoing credit evaluations of its customers' financial condition and, as deemed necessary, may require collateral, primarily letters of credit. Distributor advances, included in deferred income on shipments to distributors in the consolidated balance sheets, totaled $203.9 million at March 31, 2018 and March 31, 2017 . On sales to distributors, the Company's payment terms generally require the distributor to settle amounts owed to the Company for an amount in excess of their ultimate cost. The Company's sales price to its distributors may be higher than the amount that the distributors will ultimately owe the Company because distributors often negotiate price reductions after purchasing the products from the Company and such reductions are often significant. It is the Company's practice to apply these negotiated price discounts to future purchases, requiring the distributor to settle receivable balances, on a current basis, generally within 30 days, for amounts originally invoiced. This practice has an adverse impact on the working capital of the Company's distributors. As such, the Company has entered into agreements with certain distributors whereby it advances cash to the distributors to reduce the distributors' working capital requirements. These advances are reconciled at least on a quarterly basis and are estimated based on the amount of ending inventory as reported by the distributor multiplied by a negotiated percentage. Such advances have no impact on revenue recognition or the Company's consolidated statements of income. The Company processes discounts taken by distributors against its deferred income on shipments to distributors' balance and trues-up the advanced amounts generally after the end of each completed fiscal quarter. The terms of these advances are set forth in binding legal agreements and are unsecured, bear no interest on unsettled balances and are due upon demand. The agreements governing these advances can be canceled by the Company at any time.
Use of Estimates	Use of Estimates The Company has made a number of estimates and assumptions relating to the reporting of assets, liabilities, revenues and expenses and the disclosure of contingent assets and liabilities to prepare its consolidated financial statements in conformity with U.S. Generally Accepted Accounting Principles. Actual results could differ from those estimates.
Business Segments	Business Segments Operating segments are components of an enterprise about which separate financial information is regularly reviewed by the chief operating decision makers ("CODMs") to assess the performance of the component and make decisions about the resources to be allocated to the component. The Company's Chairman and Chief Executive Officer and the Company's President and Chief Operating Officer have been identified as the CODMs as they jointly manage the Company's worldwide consolidated enterprise. Based on the Company's structure and manner in which the Company is managed and decisions are made, the Company's business is made up of two operating segments, semiconductor products and technology licensing. In the semiconductor products segment, the Company designs, develops, manufactures and markets microcontrollers, development tools and analog, interface, mixed-signal, timing, wired and wireless connectivity devices, and memory products. Under the leadership of the CODMs, the Company is structured and organized around standardized roles and responsibilities based on product groups and functional activities. The Company's product groups are responsible for product research, design and development. The Company's functional activities include sales, marketing, manufacturing, information technology, human resources, legal and finance. The Company's product groups have similar products, production processes, types of customers and methods for distribution. In addition, the tools and technologies used in the design and manufacture of the Company's products are shared among the various product groups. The Company's product group leaders, under the direction of the CODMs, define the product roadmaps and team with sales personnel to achieve design wins and revenue and other performance targets. Product group leaders also interact with manufacturing and operational personnel who are responsible for the production, prioritization and planning of the Company's manufacturing capabilities to help ensure the efficiency of the Company's operations and fulfillment of customer requirements. This centralized structure supports a global operating strategy in which the CODMs assess performance and allocate resources based on the Company's consolidated results.
Recently Issued Accounting Pronouncements	Recently Adopted Accounting Pronouncements During the three months ended June 30, 2017, the Company adopted ASU 2015-11- Simplifying the Measurement of Inventory. This standard requires that entities measure inventory at the lower of cost and net realizable value. Net realizable value is the estimated selling price in the ordinary course of business, less reasonably predictable costs of completion, disposal, and transportation. ASU 2015-11 is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2016 and is applied prospectively. The adoption of this standard did not have a material impact on the Company's financial statements. During the three months ended June 30, 2017, the Company elected to early adopt ASU 2017-07- Compensation - Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost . This standard improves the presentation of net periodic pension cost and net periodic postretirement benefit cost. The amendment will require the employer to report the service cost component in the same line item or items as other compensation costs arising from services rendered by the pertinent employees during the period. The other components of net benefit cost will be presented separately in the income statement from the service cost component outside of income from operations. The amendment is effective for fiscal years beginning after December 15, 2017. Early adoption is permitted at the beginning of an annual period (in the first interim period) for which financial statements have not yet been issued. The adoption of this standard did not have a material impact on the Company's financial statements. During the three months ended June 30, 2016, the Company elected to early adopt ASU 2016-09- Compensation - Stock Compensation, Improvements to Employee Share-Based Payment Accounting (Topic 718), which simplifies several aspects of the accounting for share-based payment transactions. Under this standard, entities are permitted to make an accounting policy election to either estimate forfeitures on share-based payment awards, as previously required, or to recognize forfeitures as they occur. The Company elected to recognize forfeitures as they occur and the impact of that change in accounting policy was recorded as a $2.0 million cumulative effect adjustment as an increase to the Company's retained earnings and a decrease to additional paid-in capital as of April 1, 2016. The Company also recorded a cumulative-effect adjustment to retained earnings for the increase of $2.3 million in long-term deferred tax assets related to the forfeiture rate reduction on outstanding share-based payment awards. Additionally, ASU 2016-09 eliminates the requirement to report excess tax benefits and certain tax deficiencies related to share-based payment transactions in additional paid-in capital. In accordance with the new standard, the Company will record excess tax benefits and tax deficiencies as income tax benefit or provision on a prospective basis in its consolidated statements of income. The standard also eliminates the requirement that excess tax benefits be realized before companies can recognize them. Accordingly, the Company has recorded a $47.2 million cumulative-effect adjustment to its retained earnings and long-term deferred tax assets as of April 1, 2016 for previously unrecognized excess tax benefits. ASU 2016-09 also requires excess tax benefits to be reported as operating activities in the statement of cash flows rather than as a financing activity. The Company has elected to apply the change in cash flow classification on a prospective basis and prior periods were not retrospectively adjusted. Recently Issued Accounting Pronouncements Not Yet Adopted In August 2017, the FASB issued ASU 2017-12- Derivatives and Hedging (Topic 815): Targeted Improvements to Accounting for Hedging Activities . The update expands an entity's ability to apply hedge accounting for nonfinancial and financial risk components and allows for a simplified approach for fair value hedging of interest rate risk. The update eliminates the need to separately measure and report hedge ineffectiveness and generally requires the entire change in fair value of a hedging instrument to be presented in the same income statement line as the hedged item. Additionally, the update simplifies the hedge documentation and effectiveness assessment requirements under the previous guidance. The effective date of this standard is for fiscal years beginning after December 15, 2018 and early adoption is permitted. Adoption will be applied through a cumulative-effect adjustment for cash flow and net investment hedges existing at the date of adoption and prospectively for presentation and disclosure. The Company is currently evaluating the impact the adoption of this standard will have on its consolidated financial statements. In January 2017, the FASB issued ASU 2017-04- Intangibles - Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment , which simplifies the accounting for goodwill impairment. The guidance removes Step 2 of the goodwill impairment test, which requires a hypothetical purchase price allocation. A goodwill impairment will now be the amount by which a reporting unit's carrying value exceeds its fair value, not to exceed the carrying amount of goodwill. The amendment is effective for annual periods and interim periods within those annual periods, beginning after December 15, 2019, and early adoption is permitted. The Company does not expect this standard to have an impact on its consolidated financial statements. In June 2016, the FASB issued ASU 2016-13- Financial Instruments - Credit Losses: Measurement of Credit Losses on Financial Instruments. This standard requires entities to use a current lifetime expected credit loss methodology to measure impairments of certain financial assets. Using this methodology will result in earlier recognition of losses than under the current incurred loss approach, which required waiting to recognize a loss until it is probable of having been incurred. The amendments in ASU 2016-13 broaden the information that an entity must consider in developing its expected credit loss estimate for assets measured either collectively or individually and can include forecasted information. There are other provisions within the standard affecting how impairments of other financial assets may be recorded and presented, as well as expanded disclosures. ASU 2016-13 is effective for interim and annual periods beginning after December 15, 2019, and permits early adoption, but not before December 15, 2018. The standard is to be applied using a modified retrospective approach. The Company is currently evaluating the impact the adoption of this standard will have on its consolidated financial statements. In October 2016, the FASB issued ASU 2016-16- Intra-Entity Transfers of Assets Other Than Inventory . This standard addresses the recognition of current and deferred income taxes resulting from an intra-entity transfer of any asset other than inventory. Prior to the adoption of ASU 2016-16, a company will defer for financial reporting purposes the income tax expense resulting from an intra-entity asset transfer, including the taxes currently payable or paid. Upon adoption of ASU 2016-16, a company will recognize current and deferred income taxes that result from such transfers in the period in which they occur. ASU 2016-16 is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017 and is applied on a modified retrospective basis through a cumulative-effect adjustment directly to retained earnings as of the beginning of the period of adoption. The Company is currently evaluating the impact of the adoption of this standard will have on its consolidated financial statements but expects to recognize its previously deferred tax related to intra-entity transfers upon adoption of ASU 2016-16 as of April 1, 2018 with an estimated impact of up to $1.6 billion cumulative-effect increase to retained earnings. In November 2016, the FASB issued ASU 2016-18- Statement of Cash Flows: Restricted Cash. This standard requires that the statement of cash flows explain the change during the period in total cash, cash equivalents, and amounts generally described as restricted cash or restricted cash equivalents. Therefore, amounts generally described as restricted cash and restricted cash equivalents should be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. ASU 2016-18 is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted. The standard is to be applied using a retrospective transition method to each period presented. The Company is currently evaluating the impact the adoption of this standard will have on its consolidated financial statements. In February 2016, the FASB issued ASU 2016-02- Leases . This standard requires lessees to recognize a lease liability and a right-of-use asset on the balance sheet and aligns many of the underlying principles of the new lessor model with those in Accounting Standards Codification Topic 606, Revenue from Contracts with Customers. ASU 2016-02 is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018, with early adoption permitted. The standard is to be applied using the modified retrospective approach to all periods presented. The Company is currently evaluating the impact the adoption of this standard will have on its consolidated financial statements. In January 2016, the FASB issued ASU 2016-01- Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities . This standard addresses certain aspects of recognition, measurement, presentation, and disclosure of financial instruments. ASU 2016-01 is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2017. Early adoption is not permitted. The Company is currently evaluating the impact the adoption of this standard will have on its consolidated financial statements. In May 2014, the FASB issued ASU 2014-09 -Revenue from Contracts with Customers (Topic 606) , which superseded nearly all existing revenue recognition guidance under US GAAP. In August 2015, the FASB issued ASU 2015-14- Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date , which delayed the effective date of the new standard by one year to December 15, 2017, for annual and interim reporting periods beginning after that date. In accordance with the delay, the new standard was effective for the Company beginning April 1, 2018. The standard's core principle is that a company will recognize revenue when it transfers promised goods or services to customers in an amount that reflects the consideration to which the company expects to be entitled in exchange for those goods or services. The new standard allows for the amendment to be applied either retrospectively to each prior reporting period presented or retrospectively as a cumulative-effect adjustment as of the date of adoption. In March 2016, the FASB issued ASU 2016-08- Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net) , which clarifies the implementation guidance on principal versus agent considerations. In April 2016, the FASB issued ASU 2016-10- Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing , which clarifies the implementation guidance on identifying performance obligations. In May 2016, the FASB issued ASU 2016-12- Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients , which addresses implementation issues that were raised by stakeholders and discussed by the Revenue Recognition Transition Resource Group. The Company will adopt the standard under the modified retrospective transition method on April 1, 2018. As described in the Company's significant accounting policies, the Company currently defers the revenue and cost of sales on shipments to distributors until the distributor sells the product to their end customer. Upon adoption of ASU 2014-09, ASU 2015-14, ASU 2016-08, ASU 2016-10 and ASU 2016-12, the Company will no longer defer revenue until sale by the distributor to the end customer, but rather, will be required to estimate the effects of returns and allowances provided to distributors and record revenue at the time of sale to the distributor. The impact of the adoption of the standard on the Company's April 1, 2018 consolidated balance sheet is expected to include: the elimination of the entire deferred income balance as of April 1, 2018 of $334 million ; a net decrease in accounts receivable of between $41 million and $51 million ; an increase in accrued liabilities of between $10 million and $14 million ; an increase in other current assets of between $14 million and $18 million ; a decrease in inventories of between $4 million and $6 million ; and an increase in retained earnings of between $219.0 million and $259.0 million , which includes the tax impact of adoption of between $38.0 million and $58.0 million . The tax impact of adoption offsets the increase to retained earnings, and also affects the Company's deferred tax balances. The change in timing of revenue recognized for sales to distributors is the primary contributor to the expected impact on the Company's April 1, 2018 consolidated balance sheet. To a lesser extent, changes in timing of revenue recognition in the Company's licensing contracts and changes from point-in-time to over-time revenue recognition for a few of the Company's contracts also contribute to the expected impact.

Business Acquisitions (Tables)

Business Acquisitions (Tables)	12 Months Ended
	Mar. 31, 2018
Atmel Corporation
Business Acquisition [Line Items]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The table below represents the allocation of the final purchase price to the net assets acquired based on their estimated fair values, as well as the associated estimated useful lives of the acquired intangible assets (amounts in millions). Assets acquired Cash and cash equivalents $ 230.2 Accounts receivable 141.4 Inventories 335.1 Prepaid expenses and other current assets 28.4 Assets held for sale 32.0 Property, plant and equipment 129.9 Goodwill 1,286.4 Purchased intangible assets 1,888.4 Long-term deferred tax assets 46.7 Other assets 7.5 Total assets acquired 4,126.0 Liabilities assumed Accounts payable (55.7 ) Other current liabilities (121.0 ) Long-term line of credit (192.0 ) Deferred tax liabilities (27.5 ) Long-term income tax payable (115.1 ) Other long-term liabilities (141.7 ) Total liabilities assumed (653.0 ) Purchase price allocated $ 3,473.0
Schedule of Finite-Lived Intangible Assets Acquired as Part of Business Combination	Purchased Intangible Assets Weighted Average Useful Life April 4, 2016 (in years) (in millions) Core and developed technology 11 $ 1,075.0 In-process research and development — 140.7 Customer-related 6 630.6 Backlog 1 40.3 Other 5 1.8 Total purchased intangible assets $ 1,888.4
Micrel Incorporated
Business Acquisition [Line Items]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	The table below represents the allocation of the purchase price to the net assets acquired based on their estimated fair values as of August 3, 2015, as well as the associated estimated useful lives of the acquired intangible assets at that date. The purchase price allocation was finalized as of June 30, 2016 (amounts in millions): Assets acquired Cash and cash equivalents $ 99.1 Accounts receivable, net 14.0 Inventories 73.5 Prepaid expenses and other current assets 10.7 Property, plant and equipment, net 38.5 Goodwill 441.0 Purchased intangible assets 273.5 Other assets 4.3 Total assets acquired 954.6 Liabilities assumed Accounts payable (11.1 ) Other current liabilities (31.6 ) Deferred tax liabilities (88.0 ) Long-term income tax payable (7.6 ) Other long-term liabilities (0.1 ) Total liabilities assumed (138.4 ) Purchase price allocated $ 816.2
Schedule of Finite-Lived Intangible Assets Acquired as Part of Business Combination	Purchased Intangible Assets Weighted Average Useful Life August 3, 2015 (in years) (in millions) Core and developed technology 10 $ 175.8 In-process research and development — 21.0 Customer-related 5 71.1 Backlog 1 5.6 Total purchased intangible assets $ 273.5

Special Charges and Other, Net

Special Charges and Other, Net (Tables)	12 Months Ended
	Mar. 31, 2018
Other Nonrecurring (Income) Expense [Abstract]
Restructuring and Related Costs	The following is a roll forward of accrued restructuring charges for fiscal 2018 and fiscal 2017 (amounts in millions): Employee Separation Costs Exit Costs Total Balance at March 31, 2016 - Restructuring Accrual $ 0.1 $ — $ 0.1 Additions due to Atmel acquisition 6.3 — 6.3 Charges 39.1 44.1 83.2 Payments (38.9 ) (7.0 ) (45.9 ) Non-cash - Other (0.5 ) (2.3 ) (2.8 ) Changes in foreign exchange rates (0.7 ) — (0.7 ) Balance at March 31, 2017 - Restructuring Accrual 5.4 34.8 40.2 Charges 1.2 0.7 1.9 Payments (5.9 ) (9.2 ) (15.1 ) Non-cash - Other (0.2 ) 1.0 0.8 Changes in foreign exchange rates 0.3 — 0.3 Balance at March 31, 2018 - Restructuring Accrual $ 0.8 $ 27.3 $ 28.1 Current $ 11.9 Non-current 16.2 Total $ 28.1 The following table summarizes activity included in the "special charges and other, net" caption on the Company's consolidated statements of income (amounts in millions): For The Years Ended March 31, 2018 2017 2016 Restructuring Employee separation costs $ 1.2 $ 39.1 $ 9.6 Gain on sale of assets (4.4 ) — — Impairment charges — 12.6 — Contract exit costs 0.7 44.1 0.7 Other — 2.8 0.9 Legal settlement costs — — 4.3 Insurance settlement — — (11.5 ) Non-restructuring contract exit costs and other $ 20.0 $ — $ — Total $ 17.5 $ 98.6 $ 4.0

Investments (Tables)

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

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Mar. 31, 2018
Fair Value, Assets, Liabilities and Stockholders' Equity Measured on Recurring Basis [Abstract]
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation	Assets measured at fair value on a recurring basis at March 31, 2018 are as follows (amounts in millions): Quoted Prices Significant Other Observable Inputs (Level 2) Total Balance Assets Cash and cash equivalents: Money market mutual funds $ 121.0 $ — $ 121.0 Deposit accounts — 641.6 641.6 Commercial Paper — 118.7 118.7 Government agency bonds — 20.0 20.0 Short-term investments: Marketable equity securities 2.8 — 2.8 Corporate bonds and debt — 542.9 542.9 Time deposits — 11.5 11.5 Government agency bonds — 723.2 723.2 Municipal bonds - taxable — 14.9 14.9 Total assets measured at fair value $ 123.8 $ 2,072.8 $ 2,196.6 Assets measured at fair value on a recurring basis at March 31, 2017 are as follows (amounts in millions): Quoted Prices Significant Other Observable Inputs (Level 2) Total Balance Assets Cash and cash equivalents: Money market mutual funds $ 343.8 $ — $ 343.8 Deposit accounts — 564.9 564.9 Short-term investments: Marketable equity securities 1.6 — 1.6 Corporate bonds and debt — 165.2 165.2 Government agency bonds — 162.0 162.0 Municipal bonds - tax-exempt — 55.3 55.3 Municipal bonds - taxable — 10.0 10.0 Long-term Investments: Corporate bonds and debt — 42.6 42.6 Government agency bonds — 64.9 64.9 Total assets measured at fair value $ 345.4 $ 1,064.9 $ 1,410.3

Fair Value of Financial Instr39

Fair Value of Financial Instruments (Tables)	12 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value, by Balance Sheet Grouping	The following table shows the carrying amounts and fair values of the Company’s senior and junior subordinated convertible debt as of March 31, 2018 and 2017 (amounts in millions). As of March 31, 2018 and March 31, 2017 , the carrying amounts of the Company's senior and junior subordinated convertible debt have been reduced by debt issuance costs of $34.2 million and $38.3 million , respectively. March 31, 2018 2017 Carrying Amount Fair Value Carrying Amount Fair Value 2017 Senior Debt $ 1,437.6 $ 2,459.2 $ 1,384.9 $ 2,106.2 2015 Senior Debt $ 1,309.9 $ 3,079.1 $ 1,261.8 $ 2,481.7 2017 Junior Debt $ 326.7 $ 876.9 $ 262.3 $ 586.6 2007 Junior Debt $ — $ — $ 50.0 $ 445.1

Other Financial Statement Det40

Other Financial Statement Details (Tables)	12 Months Ended
	Mar. 31, 2018
Accounts Receivable, Net [Abstract]
Accounts Receivable Schedule	Accounts receivable consists of the following (amounts in millions): March 31, 2018 2017 Trade accounts receivable $ 557.8 $ 473.3 Other 8.1 7.2 Total accounts receivable, gross 565.9 480.5 Less allowance for doubtful accounts 2.2 2.1 Total accounts receivable, net $ 563.7 $ 478.4
Schedule of Inventory, Current	The components of inventories consist of the following (amounts in millions): March 31, 2018 2017 Raw materials $ 26.0 $ 14.4 Work in process 311.8 268.3 Finished goods 138.4 134.5 Total inventories $ 476.2 $ 417.2
Property, Plant and Equipment	Property, plant and equipment consists of the following (amounts in millions): March 31, 2018 2017 Land $ 73.4 $ 73.4 Building and building improvements 508.5 499.7 Machinery and equipment 1,943.9 1,774.9 Projects in process 118.3 104.3 Total property, plant and equipment, gross 2,644.1 2,452.3 Less accumulated depreciation and amortization 1,876.2 1,769.0 Total property, plant and equipment, net $ 767.9 $ 683.3

Discontinued Operations (Tables

Discontinued Operations (Tables)	12 Months Ended
	Mar. 31, 2018
Discontinued Operations and Disposal Groups [Abstract]
Disposal Groups, Including Discontinued Operations	The results of discontinued operations for the year ended March 31, 2017 are as follows (amounts in millions): March 31, 2017 Net sales $ 18.3 Cost of sales 15.8 Operating expenses 10.7 Gain on Sale 0.6 Income tax benefit (1.6 ) Net loss from discontinued operations $ (6.0 )

Intangible Assets and Goodwill

Intangible Assets and Goodwill (Tables)	12 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Intangible Assets	Intangible assets consist of the following (amounts in millions): March 31, 2018 Gross Amount Accumulated Amortization Net Amount Core and developed technology $ 1,952.3 $ (644.4 ) $ 1,307.9 Customer-related 716.9 (375.9 ) 341.0 In-process research and development 12.1 — 12.1 Distribution rights 0.3 (0.1 ) 0.2 Other 1.5 (0.7 ) 0.8 Total $ 2,683.1 $ (1,021.1 ) $ 1,662.0 March 31, 2017 Gross Amount Accumulated Amortization Net Amount Core and developed technology $ 1,932.3 $ (419.5 ) $ 1,512.8 Customer-related 716.9 (123.6 ) 593.3 Trademarks and trade names 11.7 (9.6 ) 2.1 In-process research and development 38.5 — 38.5 Distribution rights 5.6 (5.3 ) 0.3 Other 1.5 (0.4 ) 1.1 Total $ 2,706.5 $ (558.4 ) $ 2,148.1
Projected Amortization Expense	The following is an expected amortization schedule for the intangible assets for fiscal 2019 through fiscal 2023 , absent any future acquisitions or impairment charges (amounts in millions): Fiscal Year Ending March 31, Projected Amortization Expense 2019 $362.8 2020 314.6 2021 259.3 2022 192.5 2023 143.7
Goodwill Activity	Goodwill activity for fiscal 2018 and fiscal 2017 was as follows (amounts in millions): Semiconductor Products Reporting Unit Technology Licensing Reporting Unit Balance at March 31, 2016 $ 993.5 $ 19.2 Additions due to the acquisition of Atmel 1,286.3 — Balance at March 31, 2017 2,279.8 19.2 Balance at March 31, 2018 $ 2,279.8 $ 19.2

Income taxes (Tables)

Income taxes (Tables)	12 Months Ended
	Mar. 31, 2018
Income Tax Disclosure [Abstract]
Components of income tax expense (benefit)	The income tax provision consists of the following (amounts in millions): Year Ended March 31, 2018 2017 2016 Pretax Income: U.S. $ (127.3 ) $ (279.3 ) $ (75.5 ) Foreign 864.6 369.1 356.8 $ 737.3 $ 89.8 $ 281.3 Current expense (benefit): U.S. Federal $ 369.4 $ 21.3 $ (4.0 ) State 0.5 1.0 (0.2 ) Foreign 60.8 23.8 22.0 Total current $ 430.7 $ 46.1 $ 17.8 Deferred expense (benefit): U.S. Federal $ 82.5 $ (114.7 ) $ (42.2 ) State 0.1 (5.4 ) (2.0 ) Foreign (31.4 ) (6.8 ) (16.2 ) Total deferred 51.2 (126.9 ) (60.4 ) Total Income tax provision (benefit) $ 481.9 $ (80.8 ) $ (42.6 )
Reconciliation of expected federal income tax expense to actual	The sources and tax effects of the differences in the total income tax provision are as follows (amounts in millions): Year Ended March 31, 2018 2017 2016 Computed expected income tax provision $ 232.6 $ 31.4 $ 98.4 Foreign income taxed at lower than the federal rate (208.8 ) (105.0 ) (120.1 ) Impact of the Act - one-time transition tax, net of foreign tax credits 653.7 — — Impact of the Act - deferred tax effects, net of valuation allowance (136.7 ) — — Increases related to current and prior year tax positions 32.0 53.7 14.5 Decreases related to prior year tax positions (1) (11.3 ) (36.3 ) (12.1 ) Share-based compensation (27.2 ) (25.0 ) — Research and development tax credits - current year (17.0 ) (12.8 ) (13.5 ) Research and development tax credits - prior years — — (2.5 ) Intercompany prepaid tax asset amortization 7.4 7.9 (15.5 ) Withholding taxes 1.4 5.6 6.0 Foreign exchange (20.5 ) (1.7 ) 5.6 Other (3.2 ) (0.4 ) (0.9 ) Change in valuation allowance (20.5 ) 1.8 (2.5 ) Total income tax provision (benefit) $ 481.9 $ (80.8 ) $ (42.6 ) (1) The release of prior year tax positions during fiscal 2018 increased the basic and diluted net income per common share by $0.05 . The release of prior year tax positions during fiscal 2017 increased the basic and diluted net income per common share by $0.17 and $0.15 , respectively. The release of prior year tax positions during fiscal 2016 increased the basic and diluted net income per common share by $0.06 .
Schedule of deferred tax assets and liabilities	The tax effects of temporary differences that give rise to significant portions of the Company's deferred tax assets and deferred tax liabilities are as follows (amounts in millions): March 31, 2018 2017 Deferred tax assets: Deferred income on shipments to distributors $ 39.1 $ 55.7 Inventory valuation 10.7 14.6 Net operating loss carryforward 101.1 91.6 Capital loss carryforward 10.6 12.9 Share-based compensation 31.4 42.5 Income tax credits 178.4 243.0 Property, plant and equipment 25.7 59.7 Accrued expenses and other 91.2 110.4 Gross deferred tax assets 488.2 630.4 Valuation allowances (204.5 ) (210.1 ) Deferred tax assets, net of valuation allowances 283.7 420.3 Deferred tax liabilities: Convertible debt (304.4 ) (606.7 ) Intangible assets (66.6 ) (147.5 ) Other (18.3 ) (6.3 ) Deferred tax liabilities (389.3 ) (760.5 ) Net deferred tax liability $ (105.6 ) $ (340.2 ) Reported as: Non-current deferred tax assets $ 100.2 $ 68.9 Non-current deferred tax liability (205.8 ) (409.1 ) Net deferred tax liability $ (105.6 ) $ (340.2 )
Rollforward of unrecognized tax benefits	The following table summarizes the activity related to the Company's gross unrecognized tax benefits from April 1, 2015 to March 31, 2018 (amounts in millions): Year Ended March 31, 2018 2017 2016 Beginning balance $ 398.5 $ 220.7 $ 170.7 Increases related to acquisitions — 193.3 46.2 Decreases related to settlements with tax authorities (0.1 ) (11.7 ) (8.0 ) Decreases related to statute of limitation expirations (10.9 ) (7.6 ) (4.6 ) Increases related to current year tax positions 30.3 26.3 16.4 Increases (decreases) related to prior year tax positions 18.2 (22.5 ) — Ending balance $ 436.0 $ 398.5 $ 220.7

Debt and Credit Facility (Table

Debt and Credit Facility (Tables)	12 Months Ended
	Mar. 31, 2018
Convertible Debt [Abstract]
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments	Debt obligations included in the consolidated balance sheets consisted of the following (in millions): Coupon Interest Rate Effective Interest Rate Fair Value of Liability Component at Issuance (1) March 31, 2018 2017 Senior Indebtedness Credit Facility $ — $ — Senior Subordinated Convertible Debt - Principal Outstanding 2017 Senior Debt, maturing February 15, 2027 (2017 Senior Debt) 1.625% 6.0% $1,396.3 $ 2,070.0 $ 2,070.0 2015 Senior Debt, maturing February 15, 2025 (2015 Senior Debt) 1.625% 5.9% 1,160.1 1,725.0 1,725.0 Junior Subordinated Convertible Debt - Principal Outstanding 2017 Junior Debt, maturing February 15, 2037 (2017 Junior Debt) 2.250% 7.4% 321.1 686.3 575.0 2007 Junior Debt, maturing December 15, 2037 (2007 Junior Debt) 2.125% 9.1% — — 143.8 Total Convertible Debt 4,481.3 4,513.8 Gross long-term debt including current maturities 4,481.3 4,513.8 Less: Debt discount (2) (1,372.9 ) (1,516.5 ) Less: Debt issuance costs (3) (40.1 ) (46.8 ) Net long-term debt including current maturities 3,068.3 2,950.5 Less: Current maturities (4) (1,309.9 ) (50.0 ) Net long-term debt $ 1,758.4 $ 2,900.5 (1) As each of the convertible instruments may be settled in cash upon conversion, for accounting purposes, they were bifurcated into a liability component and an equity component, which are both initially recorded at fair value. The amount allocated to the equity component is the difference between the principal value of the instrument and the fair value of the liability component at issuance. The resulting debt discount is being amortized to interest expense at the respective effective interest rate over the contractual term of the debt. (2) The unamortized discount includes the following (in millions): March 31, 2018 2017 2017 Senior Debt $ (616.3 ) $ (667.5 ) 2015 Senior Debt (400.3 ) (446.6 ) 2017 Junior Debt (356.3 ) (309.3 ) 2007 Junior Debt — (93.1 ) Total unamortized discount $ (1,372.9 ) $ (1,516.5 ) (3) Debt issuance costs include the following (in millions): March 31, 2018 2017 Senior Credit Facility $ (5.9 ) $ (8.5 ) 2017 Senior Debt (16.1 ) (17.6 ) 2015 Senior Debt (14.8 ) (16.6 ) 2017 Junior Debt (3.3 ) (3.4 ) 2007 Junior Debt — (0.7 ) Total debt issuance costs $ (40.1 ) $ (46.8 ) (4) Current maturities include the full balance of the 2015 Senior Debt as of March 31, 2018 and the full balance of the 2007 Junior Debt as of March 31, 2017.
Schedule of Unamortized Discount and Debt Issuance Costs	The unamortized discount includes the following (in millions): March 31, 2018 2017 2017 Senior Debt $ (616.3 ) $ (667.5 ) 2015 Senior Debt (400.3 ) (446.6 ) 2017 Junior Debt (356.3 ) (309.3 ) 2007 Junior Debt — (93.1 ) Total unamortized discount $ (1,372.9 ) $ (1,516.5 ) (3) Debt issuance costs include the following (in millions): March 31, 2018 2017 Senior Credit Facility $ (5.9 ) $ (8.5 ) 2017 Senior Debt (16.1 ) (17.6 ) 2015 Senior Debt (14.8 ) (16.6 ) 2017 Junior Debt (3.3 ) (3.4 ) 2007 Junior Debt — (0.7 ) Total debt issuance costs $ (40.1 ) $ (46.8 )
Convertible Debt	The following table sets forth the applicable Conversion Rates adjusted for dividends declared since issuance of such series of Convertible Debt and the applicable Incremental Share Factors and Maximum Conversion Rates as adjusted for dividends paid since the applicable issuance date: Dividend adjusted rates as of March 31, 2018 Conversion Rate, adjusted Approximate Conversion Price, adjusted Incremental Share Factor, adjusted Maximum Conversion Rate, adjusted 2017 Senior Debt 10.1162 $ 98.85 5.0581 14.4156 2015 Senior Debt 15.7755 $ 63.39 7.8878 22.0857 2017 Junior Debt 10.2969 $ 97.12 5.1485 14.4156 The following is the weighted average conversion price per share used in calculating the dilutive effect (See Note 11 for details on the convertible debt): Year Ended March 31, 2018 2017 2016 2007 Junior Debt (1) $ 23.59 $ 24.01 $ 24.73 2015 Senior Debt $ 63.94 $ 65.21 $ 67.19 2017 Senior Debt $ 99.71 $ 100.58 $ — 2017 Junior Debt $ 97.96 $ 98.81 $ — (1) No longer outstanding as of December 31, 2018.
Schedule of Interest Expense	Interest expense related to convertible debt includes the following (in millions): Year Ended March 31, 2018 2017 2016 Debt issuance amortization $ 3.5 $ 2.1 $ 1.8 Amortization of debt discount - non cash interest expense 106.1 56.1 48.0 Coupon interest expense 77.3 44.5 40.2 Total $ 186.9 $ 102.7 $ 90.0

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Mar. 31, 2018
Retirement Benefits [Abstract]
Schedule of Net Benefit Costs	The aggregate net pension expense relating to these two plans is as follows (amounts in millions): Year Ended March 31, 2018 2017 Service costs $ 2.2 $ 1.4 Interest costs 1.0 1.0 Amortization of actuarial loss 0.8 — Settlements — 0.5 Net pension period cost $ 4.0 $ 2.9
Schedule of Changes in Projected Benefit Obligations	The change in projected benefit obligation and the accumulated benefit obligation, were as follows (amounts in millions): Year Ended March 31, 2018 2017 Projected benefit obligation at the beginning of the year $ 50.4 $ 40.3 Service cost 2.2 1.4 Interest cost 1.0 1.0 Settlements — 0.5 Actuarial losses 0.7 7.9 Benefits paid (0.8 ) (0.4 ) Foreign currency exchange rate changes 7.5 (0.3 ) Projected benefit obligation at the end of the year $ 61.0 $ 50.4 Accumulated benefit obligation at the end of the year $ 55.5 $ 45.6 Weighted average assumptions Discount rate 1.73 % 1.82 % Rate of compensation increase 2.91 % 2.90 %
Schedule of Assumptions Used	The change in projected benefit obligation and the accumulated benefit obligation, were as follows (amounts in millions): Year Ended March 31, 2018 2017 Projected benefit obligation at the beginning of the year $ 50.4 $ 40.3 Service cost 2.2 1.4 Interest cost 1.0 1.0 Settlements — 0.5 Actuarial losses 0.7 7.9 Benefits paid (0.8 ) (0.4 ) Foreign currency exchange rate changes 7.5 (0.3 ) Projected benefit obligation at the end of the year $ 61.0 $ 50.4 Accumulated benefit obligation at the end of the year $ 55.5 $ 45.6 Weighted average assumptions Discount rate 1.73 % 1.82 % Rate of compensation increase 2.91 % 2.90 %
Schedule of Expected Benefit Payments	Future estimated expected benefit payments for fiscal year 2019 through 2028 are as follows (amounts in millions): Fiscal Year Ending March 31, Expected Benefit Payments 2019 $ 0.9 2020 0.9 2021 1.2 2022 1.6 2023 1.6 2024 through 2028 10.6 Total $ 16.8

Equity Incentive Plans (Tables)

Equity Incentive Plans (Tables)	12 Months Ended
	Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Schedule of Share-based Compensation Expense	The following table presents the details of the Company's share-based compensation expense (amounts in millions): Year Ended March 31, 2018 2017 2016 Cost of sales $ 13.8 (1) $ 18.7 (1) $ 8.3 (1) Research and development 42.5 46.8 32.0 Selling, general and administrative 36.9 62.6 31.1 Pre-tax effect of share-based compensation 93.2 128.1 71.4 Income tax benefit 28.3 (2) 44.2 (2) 23.0 Net income effect of share-based compensation $ 64.9 $ 83.9 $ 48.4 (1) During the year ended March 31, 2018 , $ 11.9 million of share-based compensation expense was capitalized to inventory, and $ 13.8 million of previously capitalized share-based compensation expense in inventory was sold. During the year ended March 31, 2017 , $ 11.3 million of share-based compensation expense was capitalized to inventory. The amount of share-based compensation included in cost of sales during fiscal 2017 included $ 14.5 million of previously capitalized share-based compensation expense in inventory that was sold and $ 4.2 million of share-based compensation expense related to the Company's acquisition of Atmel that was not previously capitalized to inventory. During the year ended March 31, 2016 , $ 7.9 million of share-based compensation expense was capitalized to inventory, and $ 8.3 million of previously capitalized share-based compensation expense in inventory was sold. (2) Amounts exclude excess tax benefits related to share-based compensation of $ 28.1 million and $ 25.0 million for the years ended March 31, 2018 and March 31, 2017 , respectively. The Company elected to early adopt ASU 2016-09 effective April 1, 2016. Prior to the adoption of ASU 2016-09, the Company recognized excess tax benefits related to share-based compensation in additional paid-in capital. Refer to Note 1 for additional information on the adoption of this standard.
Schedule of Share-based Compensation, Restricted Stock Units Award Activity	RSU share activity under the 2004 Plan is set forth below: Number of Shares Weighted Average Grant Date Fair Value Nonvested shares at March 31, 2015 5,268,916 $ 34.15 Granted 2,479,729 38.91 Assumed upon acquisition 525,442 40.58 Forfeited (360,072 ) 38.20 Vested (1,606,273 ) 32.47 Nonvested shares at March 31, 2016 6,307,742 36.76 Granted 1,635,655 51.46 Assumed upon acquisition 2,059,524 46.57 Forfeited (722,212 ) 43.58 Vested (2,861,253 ) 38.60 Nonvested shares at March 31, 2017 6,419,456 42.06 Granted 1,267,536 77.26 Forfeited (279,051 ) 49.65 Vested (1,735,501 ) 38.00 Nonvested shares at March 31, 2018 5,672,440 $ 50.79
Schedule of Share-based Compensation, Stock Options, Activity	Stock option and stock appreciation right (SAR) activity under the Company's stock incentive plans in the three years ended March 31, 2018 is set forth below: Number of Shares Weighted Average Exercise Price per Share Outstanding at March 31, 2015 684,299 $ 28.41 Granted 244 41.09 Assumed upon acquisition 604,900 35.03 Exercised (221,987 ) 25.30 Forfeited or expired (153,948 ) 31.52 Outstanding at March 31, 2016 913,508 33.00 Exercised (437,906 ) 34.34 Forfeited or expired (42,485 ) 34.26 Outstanding at March 31, 2017 433,117 31.51 Exercised (131,666 ) 31.75 Forfeited or expired (17,111 ) 34.73 Outstanding at March 31, 2018 284,340 $ 31.21
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions	The weighted average fair values per share of stock options granted in the year ended March 31, 2016 was $ 8.85 . The fair values per share of stock options granted in the year ended March 31, 2016 were estimated utilizing the following assumptions: Year Ended March 31, 2016 Expected term (in years) 6.5 Volatility 29.50 % Risk-free interest rate 1.54 % Dividend yield 3.00 %

Commitments (Tables)

Commitments (Tables)	12 Months Ended
	Mar. 31, 2018
Commitments [Abstract]
Future minimum lease commitments	The future minimum lease commitments under these operating leases at March 31, 2018 were as follows (amounts in millions): Year Ending March 31, Amount 2019 $ 26.0 2020 18.7 2021 13.3 2022 10.2 2023 1.7 Thereafter — Total minimum payments $ 69.9

Geographic and Segment Inform48

Geographic and Segment Information (Tables)	12 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
Schedule of Revenues and Gross Profit for Each Segment	The following table represents revenues and gross profit for each segment (amounts in millions): Years ended March 31, 2018 2017 2016 Net Sales Gross Profit Net Sales Gross Profit Net Sales Gross Profit Semiconductor products $ 3,876.0 $ 2,315.9 $ 3,316.6 $ 1,666.0 $ 2,084.2 $ 1,116.4 Technology licensing 104.8 104.8 91.2 91.2 89.1 89.1 Total $ 3,980.8 $ 2,420.7 $ 3,407.8 $ 1,757.2 $ 2,173.3 $ 1,205.5
Schedule of Disclosure on Geographic Areas, Long-Lived Assets in Individual Foreign Countries by Country	Identifiable long-lived assets (consisting of property, plant and equipment net of accumulated amortization) by geographic area are as follows (amounts in millions): March 31, 2018 2017 United States $ 393.3 $ 388.5 Thailand 215.5 178.0 Various other countries 159.1 116.8 Total long-lived assets $ 767.9 $ 683.3

Derivative Instruments (Tables)

Derivative Instruments (Tables)	12 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of Fair Value Hedging Instruments, Statements of Financial Performance and Financial Position, Location	The following table summarizes the location and amount of the gain or loss on the hedged item attributable to the changes in the LIBOR swap rate and the offsetting gain or loss on the related interest rate swap agreements for the year ended March 31, 2016. The difference represents hedge ineffectiveness (amounts in millions): Year ended March 31, 2016 Income Statement Classification Gain (Loss) on 2015 Senior Debt Gain (Loss) on Interest Rate Swap Other (loss) income, net $ (18.1 ) $ 16.3

Net income per common share att

Net income per common share attributable to Microchip Technology Stockholders (Tables)	12 Months Ended
	Mar. 31, 2018
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted Earnings Per Share	The following table sets forth the computation of basic and diluted net income per common share from continuing operations attributable to Microchip stockholders (in millions, except per share amounts): Year Ended March 31, 2018 2017 2016 Net income from continuing operations attributable to Microchip Technology Stockholders $ 255.4 $ 170.6 $ 324.1 Weighted average common shares outstanding 232.9 217.2 203.4 Dilutive effect of stock options and RSUs 4.4 4.4 3.3 Dilutive effect of 2007 Junior Debt 1.3 12.7 10.7 Dilutive effect of 2015 Senior Debt 10.3 0.5 — Dilutive effect of 2017 Senior Debt — — — Dilutive effect of 2017 Junior Debt — — — Weighted average common and potential common shares outstanding 248.9 234.8 217.4 Basic net income per common share from continuing operations attributable to Microchip stockholders $ 1.10 $ 0.79 $ 1.59 Diluted net income per common share from continuing operations attributable to Microchip stockholders $ 1.03 $ 0.73 $ 1.49
Convertible Debt	The following table sets forth the applicable Conversion Rates adjusted for dividends declared since issuance of such series of Convertible Debt and the applicable Incremental Share Factors and Maximum Conversion Rates as adjusted for dividends paid since the applicable issuance date: Dividend adjusted rates as of March 31, 2018 Conversion Rate, adjusted Approximate Conversion Price, adjusted Incremental Share Factor, adjusted Maximum Conversion Rate, adjusted 2017 Senior Debt 10.1162 $ 98.85 5.0581 14.4156 2015 Senior Debt 15.7755 $ 63.39 7.8878 22.0857 2017 Junior Debt 10.2969 $ 97.12 5.1485 14.4156 The following is the weighted average conversion price per share used in calculating the dilutive effect (See Note 11 for details on the convertible debt): Year Ended March 31, 2018 2017 2016 2007 Junior Debt (1) $ 23.59 $ 24.01 $ 24.73 2015 Senior Debt $ 63.94 $ 65.21 $ 67.19 2017 Senior Debt $ 99.71 $ 100.58 $ — 2017 Junior Debt $ 97.96 $ 98.81 $ — (1) No longer outstanding as of December 31, 2018.

Quarterly Results (Unaudited) (

Quarterly Results (Unaudited) (Tables)	12 Months Ended
	Mar. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Data	The following table presents the Company's selected unaudited quarterly operating results for the eight quarters ended March 31, 2018 . The Company believes that all adjustments of a normal recurring nature have been made to present fairly the related quarterly results (in millions, except per share amounts. Amounts may not add to the total due to rounding): Fiscal 2018 First Quarter Second Quarter Third Quarter Fourth Quarter Total Net sales $ 972.1 $ 1,012.1 $ 994.2 $ 1,002.3 $ 3,980.8 Gross profit 584.4 614.1 607.1 615.1 2,420.7 Operating income 221.6 225.4 245.2 244.1 936.3 Net income (loss) attributable to Microchip Technology 170.6 189.2 (251.1 ) 146.7 255.4 Diluted net income (loss) per common share attributable to Microchip Technology stockholders 0.70 0.77 (1.07 ) 0.58 1.03 Fiscal 2017 First Quarter Second Quarter Third Quarter Fourth Quarter Total Net sales $ 799.4 $ 871.4 $ 834.4 $ 902.7 $ 3,407.8 Gross profit 348.5 410.6 465.3 532.8 1,757.2 Operating income (loss) (59.1 ) 62.8 118.1 154.1 275.8 Net income (loss) attributable to Microchip Technology (113.4 ) 33.9 107.2 136.9 164.6 Diluted net income (loss) per common share attributable to Microchip Technology stockholders (0.53 ) 0.14 0.46 0.57 0.71

Supplemental Financial Inform52

Supplemental Financial Information (Tables)	12 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Information [Abstract]
Schedule of Valuation and Qualifying Accounts Disclosure Deferred Tax Assets	A summary of additions and deductions related to the valuation allowance for deferred tax asset accounts for the years ended March 31, 2018 , 2017 and 2016 follows (amounts in millions): Balance at Beginning of Year Additions Charged to Costs and Expenses Additions Charged to Other Accounts Deductions Balance at End of Year Valuation allowance for deferred tax assets: Fiscal 2018 $ 210.1 $ 36.2 $ — $ (41.8 ) $ 204.5 Fiscal 2017 161.8 15.2 37.6 (4.5 ) 210.1 Fiscal 2016 116.5 5.5 47.8 (8.0 ) 161.8
Schedule of Valuation and Qualifying Accounts Disclosure Allowance for Doubtful Accounts	A summary of additions and deductions related to the allowance for doubtful accounts for the years ended March 31, 2018 , 2017 and 2016 follows (amounts in millions): Balance at Beginning of Year Additions Charged to Costs and Expenses Deductions (1) Balance at End of Year Allowance for doubtful accounts: Fiscal 2018 $ 2.1 $ 0.2 $ (0.1 ) $ 2.2 Fiscal 2017 $ 2.5 $ 0.2 $ (0.6 ) $ 2.1 Fiscal 2016 $ 2.6 $ 0.1 $ (0.2 ) $ 2.5 (1) Deductions represent uncollectible accounts written off, net of recoveries.
Schedule of Accumulated Other Comprehensive Income (Loss)	The following tables present the changes in the components of accumulated other comprehensive income (AOCI) for the years ended March 31, 2018 and March 31, 2017 : Unrealized Holding Gains (Losses) Available-for-sale Securities Minimum Pension Liability Foreign Currency Total Balance at March 31, 2017 $ 0.3 $ (5.3 ) $ (9.4 ) $ (14.4 ) Other comprehensive loss before reclassifications (13.6 ) (5.6 ) — (19.2 ) Amounts reclassified from accumulated other comprehensive income (loss) 15.2 0.8 — 16.0 Net other comprehensive income (loss) 1.6 (4.8 ) — (3.2 ) Balance at March 31, 2018 $ 1.9 $ (10.1 ) $ (9.4 ) $ (17.6 ) Unrealized Holding Gains (Losses) Available-for-sale Securities Minimum Pension Liability Foreign Currency Total Balance at March 31, 2016 $ 0.3 $ — $ (3.7 ) $ (3.4 ) Other comprehensive loss before reclassifications (1.5 ) (5.3 ) (5.7 ) (12.5 ) Amounts reclassified from accumulated other comprehensive income (loss) 1.5 — — 1.5 Net other comprehensive loss — (5.3 ) (5.7 ) (11.0 ) Balance at March 31, 2017 $ 0.3 $ (5.3 ) $ (9.4 ) $ (14.4 )
Reclassification out of Accumulated Other Comprehensive Income	The table below details where reclassifications of realized transactions out of AOCI are recorded on the consolidated statements of income (amounts in millions). Year ended March 31, Description of AOCI Component 2018 2017 2016 Related Statement of Income Line Unrealized (losses) gains on available-for-sale securities $ (15.5 ) $ (1.5 ) $ 10.9 Other income, net Amortization of actuarial loss (0.8 ) — — Other income, net Taxes 0.3 — — Provision for income taxes Reclassification of realized transactions, net of taxes $ (16.0 ) $ (1.5 ) $ 10.9 Net Income

SIGNIFICANT ACCOUNTING POLICI53

SIGNIFICANT ACCOUNTING POLICIES Business Acquisition (Details) $ / shares in Units, $ in Millions	Jun. 30, 2018USD ($)	Mar. 31, 2018employee	Mar. 01, 2018employee$ / shares
Microsemi
Business Acquisition [Line Items]
Business acquisition, share price (in usd per share) | $ / shares			$ 68.78
Microsemi
Business Acquisition [Line Items]
Number of employees | employee		4,800	4,800
Scenario, Forecast | Microsemi
Business Acquisition [Line Items]
Equity value acquired	$ 8,350
Enterprise value	$ 10,150

Significant Accounting Polici54

Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Mar. 31, 2018USD ($)segmentday	Mar. 31, 2017USD ($)
Accounting Policies [Abstract]
Deferred revenue on shipments to distributors	$ 479.6	$ 418
Deferred cost of sales on shipments to distributors	145.8	125.2
Deferred income on shipments to distributors	$ 333.8	292.8
Property, Plant and Equipment [Line Items]
Effective income tax rate	31.50%
Amount of distributor advances, included in deferred income on shipments to distributors in consolidated balance sheets	$ 203.9	$ 203.9
Number of reporting units	2
Number of days requiring distributor to settle receivable balances (in days) | day	30
Number of operating segments | segment	2
Minimum
Property, Plant and Equipment [Line Items]
Useful life (in years)	1 year
Minimum | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life (in years)	10 years
Minimum | Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life (in years)	3 years
Maximum
Property, Plant and Equipment [Line Items]
Useful life (in years)	15 years
Maximum | Building and Building Improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life (in years)	40 years
Maximum | Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life (in years)	7 years

Accounting Pronouncements (Deta

Accounting Pronouncements (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment			$ 49.6
Increase to other current assets	$ 55.9	$ 58.9
Decrease to inventories	(476.2)	(417.2)
Decrease to deferred taxes	(105.6)	(340.2)
Increase to retained earnings	1,397.3	$ 1,479.4
Accounting Standards Update 2016-09, Forfeiture Rate Component, Share-based Payment Award Component | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred income tax assets, net			2.3
Accounting Standards Update 2016-09, Excess Tax Benefit Component | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred income tax assets, net			47.2
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment			47.6
Retained Earnings | Accounting Standards Update 2016-09, Forfeiture Rate Component | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment			2
Retained Earnings | Accounting Standards Update 2016-09, Forfeiture Rate Component, Share-based Payment Award Component | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment			2.3
Retained Earnings | Accounting Standards Update 2016-09, Excess Tax Benefit Component | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment			47.2
Retained Earnings | Accounting Standards Update 2016-16 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment	1,600
Additional Paid-in Capital | Accounting Standards Update 2016-09, Forfeiture Rate Component | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Adoption of ASU 2016-09, cumulative adjustment			$ (2)
Calculated under Revenue Guidance in Effect before Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Elimination of deferred revenue	334
Minimum | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to accounts receivable	41
Increase to accrued liabilities	10
Increase to other current assets	14
Decrease to inventories	4
Decrease to deferred taxes	38
Increase to retained earnings	219
Maximum | Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease to accounts receivable	51
Increase to accrued liabilities	14
Increase to other current assets	18
Decrease to inventories	6
Decrease to deferred taxes	58
Increase to retained earnings	$ 259

Business Acquisitions (Narrativ

Business Acquisitions (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	Apr. 04, 2016	Aug. 03, 2015	Mar. 31, 2017	Mar. 31, 2016
Business Acquisition [Line Items]
Non-cash consideration, exchange of employee stock awards (amount)			$ 7.5	$ 4.1
Atmel Corporation
Business Acquisition [Line Items]
Cash paid for shares	$ 2,980
Shares issued in acquisition	10.1
Equity interest issued or issuable, value assigned	$ 486.1
Separately recognized transactions, expenses and losses recognized	14.9
Consideration transferred, share based payment awards	7.5
Total consideration transferred	3,470
Recognized identifiable assets acquired and liabilities assumed, liabilities	653
Payments to acquire businesses portion funded by foreign subsidiaries cash balances	2,040
Payments to acquire businesses portion funded by additional line of credit borrowings	$ 940
Useful life (in years)	9 years
Net deferred tax liability for future amortization expense of acquired intangible assets	$ 178.1
Long-term deferred tax assets	$ 46.7
Micrel Incorporated
Business Acquisition [Line Items]
Cash paid for shares		$ 430
Shares issued in acquisition		8.6
Non-cash consideration, exchange of employee stock awards (amount)		$ 4.1
Total consideration transferred		816.2
Recognized identifiable assets acquired and liabilities assumed, liabilities		138.4
Net deferred tax liability for future amortization expense of acquired intangible assets		99.7
Cash paid in a business combination for employee vested stock awards		13.1
Long-term deferred tax assets		$ 11.4

Business Acquisitions (Schedule

Business Acquisitions (Schedule of Purchase Price Allocation) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Apr. 04, 2016	Aug. 03, 2015
Business Acquisition [Line Items]
Goodwill	$ 2,299	$ 2,299
Atmel Corporation
Business Acquisition [Line Items]
Cash and cash equivalents			$ 230.2
Accounts receivable, net			141.4
Inventories			335.1
Prepaid expenses and other current assets			28.4
Assets held for sale			32
Property, plant and equipment, net			129.9
Goodwill			1,286.4
Purchased intangible assets			1,888.4
Long-term deferred tax assets			46.7
Other assets			7.5
Total assets acquired			4,126
Accounts payable			(55.7)
Other current liabilities			(121)
Long-term line of credit			(192)
Long-term income tax payable			(115.1)
Deferred tax liabilities			(27.5)
Other long-term liabilities			(141.7)
Total liabilities assumed			(653)
Purchase price allocated			$ 3,473
Micrel Incorporated
Business Acquisition [Line Items]
Cash and cash equivalents				$ 99.1
Accounts receivable, net				14
Inventories				73.5
Prepaid expenses and other current assets				10.7
Property, plant and equipment, net				38.5
Goodwill				441
Purchased intangible assets				273.5
Long-term deferred tax assets				11.4
Other assets				4.3
Total assets acquired				954.6
Accounts payable				(11.1)
Other current liabilities				(31.6)
Long-term income tax payable				(7.6)
Deferred tax liabilities				(88)
Other long-term liabilities				(0.1)
Total liabilities assumed				(138.4)
Purchase price allocated				$ 816.2

Business Acquisitions (Schedu58

Business Acquisitions (Schedule of Purchased Intangible Assets) (Details) - USD ($) $ in Millions	Apr. 04, 2016	Aug. 03, 2015	Mar. 31, 2018
Core/developed technology
Business Acquisition [Line Items]
Purchased intangible assets			$ 7.1
Atmel Corporation
Business Acquisition [Line Items]
Useful life (in years)	9 years
Purchased intangible assets	$ 1,888.4
Atmel Corporation | Core/developed technology
Business Acquisition [Line Items]
Useful life (in years)	11 years
Purchased intangible assets	$ 1,075
Atmel Corporation | Customer-related
Business Acquisition [Line Items]
Useful life (in years)	6 years
Purchased intangible assets	$ 630.6
Atmel Corporation | Backlog
Business Acquisition [Line Items]
Useful life (in years)	1 year
Purchased intangible assets	$ 40.3
Atmel Corporation | Other intangible assets
Business Acquisition [Line Items]
Useful life (in years)	5 years
Purchased intangible assets	$ 1.8
Micrel Incorporated
Business Acquisition [Line Items]
Purchased intangible assets		$ 273.5
Micrel Incorporated | Core/developed technology
Business Acquisition [Line Items]
Useful life (in years)		10 years
Purchased intangible assets		$ 175.8
Micrel Incorporated | In-process technology
Business Acquisition [Line Items]
Purchased intangible assets		$ 21
Micrel Incorporated | Customer-related
Business Acquisition [Line Items]
Useful life (in years)		5 years
Purchased intangible assets		$ 71.1
Micrel Incorporated | Backlog
Business Acquisition [Line Items]
Useful life (in years)		1 year
Purchased intangible assets		$ 5.6
In-process technology | Atmel Corporation
Business Acquisition [Line Items]
Indefinite-lived Intangible Assets Acquired	$ 140.7

Special Charges (Details)

Special Charges (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Other Nonrecurring (Income) Expense [Abstract]
Employee separation costs	$ 1.2	$ 39.1	$ 9.6
Gain on sale of assets	(4.4)	0	0
Impairment charges	0	12.6	0
Contract exit costs	0.7	44.1	0.7
Other	0	2.8	0.9
Legal settlement costs	0	0	4.3
Insurance settlement	0	0	(11.5)
Non-restructuring contract exit costs and other	20	0	0
Total	$ 17.5	$ 98.6	$ 4

Restructuring (Details)

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
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
Restructuring Charges		0.7	44.1
Payments for Restructuring		(9.2)	(7)
Restructuring Reserve, Settled without Cash		1	(2.3)
Restructuring Reserve, Foreign Currency Translation Gain (Loss)		0	0
Restructuring Reserve, Ending Balance		27.3	34.8	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
Micrel Incorporated
Restructuring Reserve [Roll Forward]
Proceeds from sale of property and assets	10
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

Summary of Available for Sale (

Summary of Available for Sale (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	$ 1,293.2	$ 501
Available-for-sale, gross unrealized gains	2.1	1
Available-for-sale, gross unrealized losses	0	(0.4)
Available-for-sale, estimated fair value	1,295.3	501.6
Government agency bonds
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	723.2	227.1
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	(0.2)
Available-for-sale, estimated fair value	723.2	226.9
Municipal bonds - tax exempt
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost		55.3
Available-for-sale, gross unrealized gains		0
Available-for-sale, gross unrealized losses		0
Available-for-sale, estimated fair value		55.3
Municipal bonds - taxable
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	14.9	10
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	14.9	10
Time deposits
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	11.5
Available-for-sale, gross unrealized gains	0
Available-for-sale, gross unrealized losses	0
Available-for-sale, estimated fair value	11.5
Corporate bonds and debt
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	542.9	207.9
Available-for-sale, gross unrealized gains	0	0.1
Available-for-sale, gross unrealized losses	0	(0.2)
Available-for-sale, estimated fair value	542.9	207.8
Equity Securities
Summary of available-for-sale securities [Line Items]
Available-for-sale, adjusted cost	0.7	0.7
Available-for-sale, gross unrealized gains	2.1	0.9
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	$ 2.8	$ 1.6

Investments AFS (Details)

Investments AFS (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 1,295.3	$ 501.6
Amount of short-term investments	1,295.3	394.1
Amount of long-term investments	0	107.5
Proceeds for sale of available-for-sale securities		470.2	$ 1,501.5
Realized gain on available-for-sale investments			$ 13.7
Impairment of Available-for-sale Securities	15.5
Corporate bonds and debt
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	542.9	207.8
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 2.8	$ 1.6

Investments Continuous Unrealiz

Investments Continuous Unrealized Loss Position (Details) $ in Millions	Mar. 31, 2017USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	$ 385
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(0.4)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position	385
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(0.4)
Government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	196.9
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(0.2)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position	196.9
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(0.2)
Municipal bonds - tax exempt
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	55.3
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position	55.3
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	0
Corporate bonds and debt
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	132.8
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(0.2)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0
Fair value of available-for-sale securities in a continuous unrealized loss position	132.8
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (0.2)

AFS, Debt Maturities (Details)

AFS, Debt Maturities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	$ 1,293.2	$ 501
Available-for-sale, gross unrealized gains	2.1	1
Available-for-sale, gross unrealized losses	0	(0.4)
Available-for-sale, estimated fair value	1,295.3	501.6
Due in one year or less
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	246.5	342.7
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	(0.2)
Available-for-sale, estimated fair value	246.5	342.5
Due after one year and through five years
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,046	157.6
Available-for-sale, gross unrealized gains	0	0.1
Available-for-sale, gross unrealized losses	0	(0.2)
Available-for-sale, estimated fair value	1,046	157.5
Due after five years and through ten years
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	0	0
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	0	0
Due after ten years
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	0	0
Available-for-sale, gross unrealized gains	0	0
Available-for-sale, gross unrealized losses	0	0
Available-for-sale, estimated fair value	0	0
Total Maturities
Schedule of available-for-sale securities, debt maturities [Line Items]
Available-for-sale, adjusted cost	1,292.5	500.3
Available-for-sale, gross unrealized gains	0	0.1
Available-for-sale, gross unrealized losses	0	(0.4)
Available-for-sale, estimated fair value	$ 1,292.5	$ 500

Measured on Recurring Basis (De

Measured on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	$ 1,295.3	$ 501.6
Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	2.8	1.6
Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	542.9	207.8
Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	11.5
Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	723.2	226.9
Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		55.3
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Commercial Paper	118.7
Total assets measured at fair value	2,196.6	1,410.3
Fair Value, Measurements, Recurring | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Government agency bonds	20
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Commercial Paper	0
Total assets measured at fair value	123.8	345.4
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Government agency bonds	0
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Commercial Paper	118.7
Total assets measured at fair value	2,072.8	1,064.9
Fair Value, Measurements, Recurring | Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Government agency bonds	20
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Money Market Funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	121	343.8
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Demand and time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	641.6	564.9
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Money Market Funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	121	343.8
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 1 | Demand and time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Money Market Funds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	0	0
Fair Value, Measurements, Recurring | Cash and Cash Equivalents | Fair Value, Inputs, Level 2 | Demand and time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Cash and cash equivalents	641.6	564.9
Fair Value, Measurements, Recurring | Short-term Investments | Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	2.8	1.6
Fair Value, Measurements, Recurring | Short-term Investments | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	542.9	165.2
Fair Value, Measurements, Recurring | Short-term Investments | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	11.5
Fair Value, Measurements, Recurring | Short-term Investments | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	723.2	162
Fair Value, Measurements, Recurring | Short-term Investments | Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		55.3
Fair Value, Measurements, Recurring | Short-term Investments | Municipal bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	14.9	10
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	2.8	1.6
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0	0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0	0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 1 | Municipal bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0	0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Equity Securities
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	0	0
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	542.9	165.2
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Time deposits
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	11.5
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	723.2	162
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds- tax exempt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		55.3
Fair Value, Measurements, Recurring | Short-term Investments | Fair Value, Inputs, Level 2 | Municipal bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities	$ 14.9	10
Fair Value, Measurements, Recurring | Other Long-term Investments | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		42.6
Fair Value, Measurements, Recurring | Other Long-term Investments | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		64.9
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 1 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		0
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 1 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		0
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 2 | Corporate bonds and debt
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		42.6
Fair Value, Measurements, Recurring | Other Long-term Investments | Fair Value, Inputs, Level 2 | Government agency bonds
Fair Value, Assets and Liabilities, as Classified on the Condensed Consolidated Balance Sheets [Abstract]
Available-for-sale Securities		$ 64.9

Fair Value of Financial Instr66

Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unamortized debt issuance expense	$ 34.2	$ 38.3
Fair Value | 2017 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	2,459.2	2,106.2
Fair Value | 2015 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	3,079.1	2,481.7
Fair Value | 2017 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	876.9	586.6
Fair Value | 2007 Junior Subordinated Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	0	445.1
Carrying Amount | 2017 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	1,437.6	1,384.9
Carrying Amount | 2015 Senior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	1,309.9	1,261.8
Carrying Amount | 2017 Junior Subordinated Convertible Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	326.7	262.3
Carrying Amount | 2007 Junior Subordinated Convertible Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value disclosure for Subordinated Convertible Debentures	$ 0	$ 50

Other Financial Statement Det67

Other Financial Statement Details (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivable amounts	$ 565.9	$ 480.5
Less allowance for doubtful accounts	2.2	2.1
Accounts receivable, net	563.7	478.4
Trade Accounts Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivable amounts	557.8	473.3
Other Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivable amounts	$ 8.1	$ 7.2

Inventories (Details)

Inventories (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Receivables [Abstract]
Raw materials	$ 26	$ 14.4
Work in process	311.8	268.3
Finished goods	138.4	134.5
Total inventories	$ 476.2	$ 417.2

Property, Plant and Equipment (

Property, Plant and Equipment (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	$ 2,644.1		$ 2,644.1	$ 2,452.3
Less accumulated depreciation and amortization	1,876.2		1,876.2	1,769
Total property, plant and equipment, net	767.9		767.9	683.3
Depreciation			123.7	122.9	$ 103.9
Land
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	73.4		73.4	73.4
Building and Building Improvements
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	508.5		508.5	499.7
Machinery and equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	1,943.9		1,943.9	1,774.9
Construction in Progress
Property, Plant and Equipment [Line Items]
Property, plant and equipment, gross	118.3		$ 118.3	$ 104.3
Building
Property, Plant and Equipment [Line Items]
Purchase of building	$ 40.8
Micrel Incorporated
Property, Plant and Equipment [Line Items]
Proceeds from sale of property and assets		$ 10

Discontinued Operations (Detail

Discontinued Operations (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Discontinued Operations and Disposal Groups [Abstract]
Net sales		$ 18.3
Cost of sales		15.8
Operating expenses		10.7
Gain on Sale		0.6
Income tax benefit	$ 0	(1.6)	$ 0
Net loss from discontinued operations	$ 0	$ (6)	$ 0

Intangible Assets, by Major Cla

Intangible Assets, by Major Class (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 2,683.1	$ 2,706.5
Accumulated Amortization	(1,021.1)	(558.4)
Net Amount	1,662	2,148.1
In-Process Technology Reaching Technological Feasibility And Reclassified	26.2
Projected Amortization Expense
2,019	362.8
2,020	314.6
2,021	259.3
2,022	192.5
2,023	143.7
Amortization of acquired intangible assets	492.2	346.3	$ 179.3
Impairment of intangible assets	0.5	11.9	0.6
Cost of Sales
Projected Amortization Expense
Amortization of acquired intangible assets	6.1	4	3.6
Operating Expense
Projected Amortization Expense
Amortization of acquired intangible assets	$ 486.1	342.3	$ 175.7
Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life (in years)	1 year
Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life (in years)	15 years
Core and developed technology
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 1,952.3	1,932.3
Accumulated Amortization	(644.4)	(419.5)
Net Amount	1,307.9	1,512.8
Purchased intangible assets	$ 7.1
Acquired finite-lived intangible assets, weighted average useful life (in years)	9 years
Customer-related
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 716.9	716.9
Accumulated Amortization	(375.9)	(123.6)
Net Amount	341	593.3
Trademarks and trade names
Finite-Lived Intangible Assets [Line Items]
Gross Amount		11.7
Accumulated Amortization		(9.6)
Net Amount		2.1
In-process research and development
Finite-Lived Intangible Assets [Line Items]
Gross Amount	12.1	38.5
Accumulated Amortization	0	0
Net Amount	12.1	38.5
Distribution rights
Finite-Lived Intangible Assets [Line Items]
Gross Amount	0.3	5.6
Accumulated Amortization	(0.1)	(5.3)
Net Amount	0.2	0.3
Other intangible assets
Finite-Lived Intangible Assets [Line Items]
Gross Amount	1.5	1.5
Accumulated Amortization	(0.7)	(0.4)
Net Amount	$ 0.8	$ 1.1

Goodwill by Reporting Segment (

Goodwill by Reporting Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2018	Apr. 04, 2016
Goodwill [Roll Forward]
Goodwill, ending balance	$ 2,299
Goodwill	2,299	$ 2,299
Atmel Corporation
Goodwill [Roll Forward]
Goodwill			$ 1,286.4
Semiconductor products
Goodwill [Roll Forward]
Goodwill, beginning balance	993.5
Goodwill, ending balance	2,279.8
Goodwill	993.5	2,279.8
Semiconductor products | Atmel Corporation
Goodwill [Roll Forward]
Additions due to business combination	1,286.3
Technology licensing
Goodwill [Roll Forward]
Goodwill, beginning balance	19.2
Goodwill, ending balance	19.2
Goodwill	19.2	$ 19.2
Technology licensing | Atmel Corporation
Goodwill [Roll Forward]
Additions due to business combination	$ 0

Income tax (benefit) provision

Income tax (benefit) provision from continuing operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Results of Operations, Income before Income Taxes [Abstract]
U.S.	$ (127.3)	$ (279.3)	$ (75.5)
Foreign	864.6	369.1	356.8
Income before income taxes	737.3	89.8	281.3
Current expense (benefit):
U.S. Federal	369.4	21.3	(4)
State	0.5	1	(0.2)
Foreign	60.8	23.8	22
Total current	430.7	46.1	17.8
Deferred expense (benefit):
U.S. Federal	82.5	(114.7)	(42.2)
State	0.1	(5.4)	(2)
Foreign	(31.4)	(6.8)	(16.2)
Total deferred	51.2	(126.9)	(60.4)
Total Income tax provision (benefit)	481.9	(80.8)	(42.6)
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Computed expected income tax provision	232.6	31.4	98.4
Foreign income taxed at lower than the federal rate	(208.8)	(105)	(120.1)
Impact of the Act - one-time transition tax, net of foreign tax credits	653.7	0	0
Impact of the Act - deferred tax effects, net of valuation allowance	(136.7)	0	0
Impact of the Act - deferred tax effects, net of valuation allowance	32	53.7	14.5
Decreases related to prior year tax positions (1)	(11.3)	(36.3)	(12.1)
Decreases related to prior year tax positions (1)	(27.2)	(25)	0
Research and development tax credits - current year	(17)	(12.8)	(13.5)
Research and development tax credits - current year	0	0	(2.5)
Intercompany prepaid tax asset amortization	7.4	7.9	(15.5)
Withholding taxes	1.4	5.6	6
Foreign exchange	(20.5)	(1.7)	5.6
Other	(3.2)	(0.4)	(0.9)
Change in valuation allowance	(20.5)	1.8	(2.5)
Income tax provision (benefit)	$ 481.9	$ (80.8)	$ (42.6)
Increase of basic net income per share due to release of prior year tax positions	$ 0.05	$ 0.17	$ 0.06
Increase of diluted net income per share due to release of prior year tax positions	$ 0.05	$ 0.15	$ 0.06
Income tax holiday, aggregate dollar amount	$ 6.2	$ 13.2	$ 9.4
Income tax holiday, benefits per share, basic	$ 0.03	$ 0.06	$ 0.05
Income tax holiday, benefits per share, diluted	$ 0.02	$ 0.06	$ 0.04

INCOME TAXES Tax Cuts and Jobs

INCOME TAXES Tax Cuts and Jobs Act (Details) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Effective income tax rate		31.50%
Tax Cuts And Jobs Act Of 2017, transition tax		$ 471.6
Tax Cuts And Jobs Act Of 2017, effect of change on earnings per share, diluted (in usd per share)		$ 1.89
Repatriation of foreign earnings		$ 653.7	$ 0	$ 0
Tax Cuts And Jobs Act Of 2017, provisional income tax expense (benefit)	$ 644.7
Tax Cuts And Jobs Act Of 2017, Change in tax rate, provisional income tax expense	9
Tax Cuts And Jobs Act Of 2017, provisional income tax benefit	$ 136.7
Tax Cuts And Jobs Act Of 2017, provisional amount recorded for valuation allowance release		$ (36.4)

Components of deferred tax asse

Components of deferred tax assets and liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017
Components of Deferred Tax Assets [Abstract]
Deferred income on shipments to distributors	$ 39.1	$ 55.7
Inventory valuation	10.7	14.6
Net operating loss carryforward	101.1	91.6
Capital loss carryforward	10.6	12.9
Share-based compensation	31.4	42.5
Income tax credits	178.4	243
Property, plant and equipment	25.7	59.7
Accrued expenses and other	91.2	110.4
Gross deferred tax assets	488.2	630.4
Valuation allowances	(204.5)	(210.1)
Deferred tax assets, net of valuation allowances	283.7	420.3
Components of Deferred Tax Liabilities [Abstract]
Convertible debt	(304.4)	(606.7)
Intangible assets	(66.6)	(147.5)
Other	(18.3)	(6.3)
Deferred Tax Liabilities, Gross	(389.3)	(760.5)
Net deferred tax liability	(105.6)	(340.2)
Non-current deferred tax assets	100.2	68.9
Non-current deferred tax liability	$ (205.8)	$ (409.1)

Operating loss carryforwards (D

Operating loss carryforwards (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Loss Carryforwards [Line Items]
Capital loss carryforward	$ 10.6	$ 12.9
Capital loss carryforward, expiration date	Mar. 31, 2020
Capital loss carryforward, valuation allowance	$ 10.4
Foreign Tax Authority and Federal and State Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	101.1
Operating loss carryforwards, valuation allowance	$ 74.6
Minimum | Foreign Tax Authority and Federal and State Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration date	Mar. 31, 2018
Maximum | Foreign Tax Authority and Federal and State Jurisdiction
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards, expiration date	Mar. 31, 2037

Tax credit carryforward (Detail

Tax credit carryforward (Details) $ in Millions	12 Months Ended
	Mar. 31, 2018USD ($)
Tax Credit Carryforward [Line Items]
Refundable foreign tax credits	$ 49.2
Foreign Tax Authority | Foreign Withholding Tax Credit
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	20
Tax credit carryforward, valuation allowance	$ 20
Foreign Tax Authority | Foreign Withholding Tax Credit | Minimum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2022
Foreign Tax Authority | Foreign Withholding Tax Credit | Maximum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2024
State and Local Jurisdiction
Tax Credit Carryforward [Line Items]
Tax credit carryforward, amount	$ 96.3
Tax credit carryforward, valuation allowance	$ 96.3
State and Local Jurisdiction | Minimum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2018
State and Local Jurisdiction | Maximum
Tax Credit Carryforward [Line Items]
Tax credit carryforward, expiration date	Mar. 31, 2037

Unrecognized tax benefits (Deta

Unrecognized tax benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning balance	$ 436	$ 398.5	$ 220.7	$ 170.7
Increases related to acquisitions		0	193.3	46.2
Decreases related to settlements with tax authorities		(0.1)	(11.7)	(8)
Decreases related to statute of limitation expirations		(10.9)	(7.6)	(4.6)
Increases related to current year tax positions		30.3	26.3	16.4
Increases (decreases) related to prior year tax positions			(22.5)
Increases related to prior year tax positions		18.2		0
Ending balance		436	398.5	220.7
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued [Abstract]
Unrecognized tax benefits, interest on income taxes accrued		12.9	9.4
Unrecognized tax benefits, income tax penalties accrued		67.9	66.1
Unrecognized tax benefits, income tax penalties and interest expense		$ 5.4	$ 5.8	$ 1.7
Scenario, Forecast
Income Tax Contingency [Line Items]
Unrecognized tax benefits, potential decrease	$ 20

Debt Obligations (Details)

Debt Obligations (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Long-term debt, gross	$ 4,481.3	$ 4,513.8
Total Convertible Debt	4,481.3	4,513.8
Amount of unamortized debt discount of debentures	(1,372.9)	(1,516.5)
Debt issuance amortization	(40.1)	(46.8)	$ (33.7)
Net long-term debt including current maturities	3,068.3	2,950.5
Less: Current maturities	(1,309.9)	(50)
Net long-term debt	1,758.4	2,900.5
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Debt Instrument [Line Items]
Credit Facility	0	0
Debt issuance amortization	$ (5.9)	(8.5)
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	1.625%
Debt Instrument, Interest Rate, Effective Percentage	5.97%
Estimated Fair Value Of Liability Component Of Debentures On Issuance Date	$ 1,396.3
Long-term debt, gross	2,070	2,070
Amount of unamortized debt discount of debentures	(616.3)	(667.5)
Debt issuance amortization	$ (16.1)	(17.6)
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	1.625%
Debt Instrument, Interest Rate, Effective Percentage	5.90%
Estimated Fair Value Of Liability Component Of Debentures On Issuance Date	$ 1,160.1
Long-term debt, gross	1,725	1,725
Amount of unamortized debt discount of debentures	(400.3)	(446.6)
Debt issuance amortization	$ (14.8)	(16.6)		$ (30.3)
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Coupon Interest Rate	2.25%
Debt Instrument, Interest Rate, Effective Percentage	7.37%
Estimated Fair Value Of Liability Component Of Debentures On Issuance Date	$ 321.1
Long-term debt, gross	686.3	575
Amount of unamortized debt discount of debentures	(356.3)	(309.3)
Debt issuance amortization	$ (3.3)	(3.4)
2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Coupon Interest Rate	2.125%
Debt Instrument, Interest Rate, Effective Percentage	9.10%
Estimated Fair Value Of Liability Component Of Debentures On Issuance Date	$ 0
Long-term debt, gross	0	143.8
Amount of unamortized debt discount of debentures	0	(93.1)
Debt issuance amortization	$ 0	$ (0.7)

Components (Details)

Components (Details) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 31, 2017	Feb. 28, 2017	Feb. 28, 2015
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	$ (1,372.9)	$ (1,516.5)
Debt issuance cost	(40.1)	(46.8)	$ (33.7)
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Debt Instrument [Line Items]
Debt issuance cost	(5.9)	(8.5)
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(616.3)	(667.5)
Debt issuance cost	(16.1)	(17.6)
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(400.3)	(446.6)
Debt issuance cost	(14.8)	(16.6)		$ (30.3)
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	(356.3)	(309.3)
Debt issuance cost	(3.3)	(3.4)
2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Amount of unamortized debt discount of debentures	0	(93.1)
Debt issuance cost	$ 0	$ (0.7)

Conversion Features (Details)

Conversion Features (Details) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2018USD ($)day
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Threshold percentage of stock price trigger	130.00%
If-converted value in excess of principal | $		$ 1,141.7
Convertible Debt
Debt Instrument [Line Items]
Debt Instrument, Redemption Price, Percentage	100.00%
Convertible Debt | Debt Instrument, Redemption, Period One
Debt Instrument [Line Items]
Threshold percentage of stock price trigger		130.00%
Threshold trading days		20
Threshold consecutive trading days		30
Convertible Debt | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Threshold percentage of stock price trigger		98.00%
Threshold trading days		5
Threshold consecutive trading days		10

Convertible Debt (Details)

Convertible Debt (Details)	12 Months Ended
	Mar. 31, 2018$ / shares
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Conversion Rate, adjusted	10.1162
Approximate Conversion Price, adjusted	$ 98.85
Incremental Share Factor, adjusted	5.0581
Maximum Conversion Rate, adjusted	14.4156
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Conversion Rate, adjusted	15.7755
Approximate Conversion Price, adjusted	$ 63.39
Incremental Share Factor, adjusted	7.8878
Maximum Conversion Rate, adjusted	22.0857
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Conversion Rate, adjusted	10.2969
Approximate Conversion Price, adjusted	$ 97.12
Incremental Share Factor, adjusted	5.1485
Maximum Conversion Rate, adjusted	14.4156

Amortization and Interest Expen

Amortization and Interest Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Debt Instrument [Line Items]
Amortization of debt issuance costs	$ 6.6	$ 4.5	$ 4
Amortization of debt discount on convertible debt	106.1	56.1	48
Convertible Debt
Debt Instrument [Line Items]
Amortization of debt issuance costs	3.5	2.1	1.8
Amortization of debt discount on convertible debt	106.1	56.1	48
Coupon interest expense	77.3	44.5	40.2
Total	$ 186.9	$ 102.7	$ 90
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)	8 years 10 months 16 days
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)	6 years 10 months 16 days
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Remaining period, in years, over which unamortized debt discount will be recognized as non-cash interest expense (in years)	18 years 10 months 16 days

Debt and Credit Facility (Detai

Debt and Credit Facility (Details) - USD ($) $ in Millions	1 Months Ended				3 Months Ended			12 Months Ended
	Nov. 30, 2017	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Debt Instrument [Line Items]
Debt issuance amortization			$ 33.7				$ 46.8	$ 40.1	$ 46.8
Debt portion of convertible debt issuance costs			3.4
Equity portion of convertible debt issuance costs			12.5
Induced conversion of convertible debt expense			5
Shares issued to settle 2007 junior debt, value								298.5	862.7
Loss on settlement of convertible debt					$ 2.2	$ 13.8	43.9	16	43.9	$ 0	$ 50.6
Amount of unamortized debt discount of debentures							1,516.5	1,372.9	1,516.5
2015 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs				$ 1,694.7
Debt issuance amortization				30.3			16.6	14.8	16.6
Debt portion of convertible debt issuance costs				20.4
Equity portion of convertible debt issuance costs				9.9
Amount of unamortized debt discount of debentures							446.6	400.3	446.6
2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount								143.8
Extinguishment of debt, principal amount								$ 32.5
Shares converted								500,000
Debt issuance amortization							0.7	$ 0	0.7
Shares issued to settle convertible debt								3,700,000
Loss on settlement of convertible debt		$ 13.8						$ 2.2
Amount of unamortized debt discount of debentures							93.1	0	93.1
2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			567.7
Debt issuance amortization							3.4	3.3	3.4
Amount of unamortized debt discount of debentures							309.3	356.3	309.3
2017 Senior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs			2,043.6
Debt issuance amortization							17.6	16.1	17.6
Debt portion of convertible debt issuance costs			17.8
Amount of unamortized debt discount of debentures							$ 667.5	616.3	$ 667.5
Long-term Debt | 2007 Junior Subordinated Convertible Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 14.6	$ 111.3	$ 431.3	575				$ 111.3
Shares issued to settle convertible debt		3,200,000	11,997,924					3,200,000
Payment for Debt Extinguishment or Debt Prepayment Cost								$ 41
Shares issued to settle 2007 junior debt, value		$ 254.6	$ 862.7
Extinguishment of debt		374	1,293.9	1,134.6
Extinguishment of debt, liability component		56.3	188	238.3
Extinguishment of debt, equity component		321.1	$ 1,105.9	$ 896.3
Long-term Debt | 2017 Junior Subordinated Convertible Debt
Debt Instrument [Line Items]
Proceeds from issuance of senior subordinated convertible debentures, net of issuance costs		111.3
Shares issued to settle 2007 junior debt, value		$ 119.3
Amount of unamortized debt discount of debentures								$ 55.1

Credit Facility (Details)

Credit Facility (Details)	1 Months Ended	12 Months Ended
	Feb. 28, 2017USD ($)	Mar. 31, 2018USD ($)	Mar. 31, 2017USD ($)	Mar. 31, 2016USD ($)	Jun. 30, 2017USD ($)
Line of Credit Facility [Line Items]
Interest coverage ratio, less than		3.50
Debt issuance amortization	$ 33,700,000	$ 40,100,000	$ 46,800,000
Description of variable rate basis		the highest of JPMorgan Chase Bank, N.A.'s prime rate, the federal funds rate plus a margin equal to 0.50% and the adjusted LIBOR rate for a 1-month interest period plus a margin equal to 1.00%
Interest expense		$ 199,000,000	146,300,000	$ 104,000,000
Foreign Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		125,000,000
Standby Letters of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		25,000,000
Line of Credit Facility Swingline Loan Sublimit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		25,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 2,774,000,000
Credit facility		$ 0
Period for leverage ratio to determine interest rate spread		preceding four fiscal quarters
Interest expense		$ 9,700,000	42,900,000
Interest rate spread on overdue principal in event of default		2.00%
Interest rate spread on overdue amounts in event of default		2.00%
2020 Tranche | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 3,122,000,000
February 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Line of Credit Facility [Line Items]
Interest expense				$ 18,900,000
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020
Line of Credit Facility [Line Items]
Debt issuance amortization		5,900,000	8,500,000
Credit facility		$ 0	$ 0
December 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Total leverage ratio, maximum		5
Total consolidated debt, maximum		$ 700,000,000
Total leverage ratio, temporary maximum allowed, first four quarters		5.50
Total leverage ratio, temporary maximum allowed, next three quarters		5.25
Senior leverage ratio, maximum		3.50
Senior leverage ratio, minimum to obtain additional funding			2.50
Repayments of lines of credit	1,682,500,000
Debt issuance amortization	$ 2,100,000
Debt Instrument Interest Rate Option 2 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Description of variable rate basis		adjusted LIBOR rate (based on one, two, three, or six-month interest periods)
Minimum | Debt Instrument Interest Rate Option 1 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate		0.25%
Maximum | Debt Instrument Interest Rate Option 1 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate		1.25%
Maximum | Debt Instrument Interest Rate Option 1 | February 2015 Amended Credit Facility with Maturity Dates of June 2018 and February 2020 | Line of Credit
Line of Credit Facility [Line Items]
Basis spread on variable rate		1.25%
Maximum | Debt Instrument Interest Rate Option 2 | Revolving Credit Facility
Line of Credit Facility [Line Items]
Basis spread on variable rate		2.25%

Contingencies (Details)

Contingencies (Details) $ in Millions	Mar. 31, 2018USD ($)
Loss contingencies [Line Items]
Loss contingencies, estimate of possible loss	$ 100
Damages from product defects
Loss contingencies [Line Items]
Loss contingencies, estimate of possible loss	227.7
Indemnification agreement
Loss contingencies [Line Items]
Loss contingencies, estimate of possible loss	$ 160.5

Stock Repurchase Activity (Deta

Stock Repurchase Activity (Details) - USD ($) $ in Millions	12 Months Ended	88 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Jan. 18, 2016	May 15, 2015	Dec. 11, 2007
Equity [Abstract]
Stock repurchase program, number of shares authorized to be repurchased			15,000,000	20,000,000	10,000,000
Repurchased shares under authorization (in shares)	8,600,000	7,500,000
Amount of repurchased shares under authorization	$ 363.8	$ 234.7
Stock repurchase program, remaining number of shares authorized to be repurchased				11,400,000	2,500,000

Schedule of Net Pension Expense

Schedule of Net Pension Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Retirement Benefits [Abstract]
Service costs	$ 2.2	$ 1.4
Interest costs	1	1
Amortization of actuarial loss	(0.8)	0
Settlements	0	0.5
Net pension period cost	$ 4	$ 2.9

Change in Obligation (Details)

Change in Obligation (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Projected benefit obligation at the beginning of the year	$ 50.4	$ 40.3
Service costs	2.2	1.4
Interest costs	1	1
Settlements	0	0.5
Actuarial losses	0.7	7.9
Benefits paid	(0.8)	(0.4)
Foreign currency exchange rate changes	7.5	(0.3)
Projected benefit obligation at the end of the year	61	50.4
Accumulated benefit obligation at the end of the year	55.5	45.6
Defined benefit pension plan, liabilities	61	50.4
Defined benefit pension plan liabilities, current	0.9	0.7
Defined benefit pension plan, liabilities, noncurrent	$ 60.1	$ 49.7

Schedule of Assumptions (Detail

Schedule of Assumptions (Details)	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Retirement Benefits [Abstract]
Weighted average assumed discount rate	1.73%	1.82%
Weighted average assumed compensation rate of increase	2.91%	2.90%

Expected Future Payments (Detai

Expected Future Payments (Details) $ in Millions	Mar. 31, 2018USD ($)
Retirement Benefits [Abstract]
2,019	$ 0.9
2,020	0.9
2,021	1.2
2,022	1.6
2,023	1.6
2024-2028	10.6
Total	$ 16.8

Employee Benefit Plans (Details

Employee Benefit Plans (Details) - USD ($) $ in Millions	Jan. 01, 2017	Jan. 01, 2016	Jan. 01, 2015	May 01, 2006	Jan. 01, 2005	Mar. 01, 2002	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 1995
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Net periodic benefit cost expected In next fiscal year							$ 3.1
Defined benefit plan, benefit obligation							61	$ 50.4	$ 40.3
Defined benefit plan, expected future benefit payments, next twelve months							0.9
Overlapping Stock Purchase Period 1
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Overlapping period on purchase of common stock under purchase plan (in number of months)						24 months
Overlapping Stock Purchase Period 2
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Overlapping period on purchase of common stock under purchase plan (in number of months)						18 months
Overlapping Stock Purchase Period 3
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Overlapping period on purchase of common stock under purchase plan (in number of months)						12 months
Overlapping Stock Purchase Period 4
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Overlapping period on purchase of common stock under purchase plan (in number of months)						6 months
Supplemental employee retirement plans, defined benefit
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Defined benefit plan, benefit obligation							4
Defined benefit plan, expected future benefit payments, next twelve months							0.4
Defined benefit plan, expected future benefit payments, year two through year ten							$ 3.5
Defined contribution pension
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Percentage of employee's base salary allowed as employee contribution (in hundredths)							60.00%
Amount of company contributions							$ 8.8	8.2	4.4
Employee stock purchase plan, 2001 plan
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Percentage of fair market value of common stock for employee purchase (in hundredths)						85.00%
Number of additional shares reserved under purchase plan (in shares)	1,077,150	1,017,492	0
Number of shares of common stock reserved for issuance (in shares)							13,372,504
Shares issued under purchase plan (in shares)							7,895,340
Employee stock purchase plan, 2001 plan | Maximum
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Maximum automatic increase in number of common stock shares under purchase plan (in shares)					1,500,000
Percentage of outstanding shares of common stock used to determine automatic annual increase in common stock (in hundredths)					0.50%
Cash bonus plan
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Compensation expense charged against operations							$ 36.3	28.2	14.2
Management incentive compensation plan
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Compensation expense charged against operations							$ 48.1	$ 41.5	$ 19.1
Employee Stock Purchase Plan Employees In Non Us Locations
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Percentage of fair market value of common stock for employee purchase (in hundredths)				85.00%
Number of additional shares reserved under purchase plan (in shares)	215,430	203,498	0
Number of shares of common stock reserved for issuance (in shares)							1,919,213
Shares issued under purchase plan (in shares)							1,347,972
Percentage of fair market value of common stock at the beginning or end of semi-annual purchase plan period, during fiscal 1995 (in hundredths)										100.00%
Employee Stock Purchase Plan Employees In Non Us Locations | Maximum
Deferred compensation arrangement, by type of deferred compensation [Line Items]
Percentage of outstanding shares of common stock used to determine automatic annual increase in common stock (in hundredths)					0.10%

Equity Incentive Plans, Stock C

Equity Incentive Plans, Stock Compensation Expense (Details) - USD ($) $ in Millions	Apr. 04, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Stock-based compensation expense [Line Items]
Share based compensation expense		$ 93.2	$ 128.1	$ 71.4
Income tax benefit		28.3	44.2	23
Net income effect of share-based compensation		64.9	83.9	48.4
Share-based compensation expense previously capitalized		13.8		8.3
Total compensation cost not yet recognized		$ 169.6
Nonvested awards, total compensation cost not yet recognized, period for recognition		1 year 10 months 28 days
Inventory
Stock-based compensation expense [Line Items]
Income tax benefit		$ 11.9	11.3	7.9
Share-based compensation expense previously capitalized			14.5
Cost of Sales
Stock-based compensation expense [Line Items]
Share based compensation expense		13.8	18.7	8.3
Research and Development Expense
Stock-based compensation expense [Line Items]
Share based compensation expense		42.5	46.8	32
Selling General And Administrative
Stock-based compensation expense [Line Items]
Share based compensation expense		36.9	62.6	$ 31.1
Atmel Corporation
Stock-based compensation expense [Line Items]
Share-based compensation expense previously capitalized			4.2
Total compensation cost not yet recognized	$ 88.4
Business combination fair value of assumed awards at acquisition date	95.9
Consideration transferred, share based payment awards	$ 7.5
Accounting Standards Update 2016-09, Excess Tax Benefit Component | New Accounting Pronouncement, Early Adoption, Effect
Stock-based compensation expense [Line Items]
Share based compensation expense		$ 28.1	$ 25

Equity Incentive Plans, Share-b

Equity Incentive Plans, Share-based Payment Award (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Closing price of company's common stock related to calculation of aggregate intrinsic value of stock options (in dollars per share)	$ 91.36
Restricted stock units' weighted average remaining expense recognition period (in years)	1 year 10 months 28 days
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
RSU nonvested shares beginning balance	6,419,456	6,307,742	5,268,916
RSU shares granted	1,267,536	1,635,655	2,479,729
RSU shares assumed upon acquisition		2,059,524	525,442
RSU shares forfeited	(279,051)	(722,212)	(360,072)
RSU shares vested	(1,735,501)	(2,861,253)	(1,606,273)
RSU nonvested shares ending balance	5,672,440	6,419,456	6,307,742
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Weighted average grant date fair values per share of restricted stock, beginning balance	$ 42.06	$ 36.76	$ 34.15
Weighted average grant date fair values per share of restricted stock granted in period	77.26	51.46	38.91
Weighted average grant date fair values per share of restricted stock units acquired		46.57	40.58
Weighted average grant date fair values per share of restricted stock units forfeited	49.65	43.58	38.20
Weighted average fair values per share of restricted stock units awarded	38	38.60	32.47
Weighted average grant date fair values per share of restricted stock, ending balance	$ 50.79	$ 42.06	$ 36.76
Total intrinsic value of RSU's vested during the period	$ 146	$ 166.1	$ 72.1
Total intrinsic value of RSU's outstanding	$ 518.2
Restricted stock units' weighted average remaining expense recognition period (in years)	1 year 11 months 16 days

Equity Incentive Plans, Stock O

Equity Incentive Plans, Stock Options Outstanding (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Option shares outstanding beginning balance (in shares)	433,117	913,508	684,299
Options granted (in shares)			244
Options assumed upon acquisition (in shares)			604,900
Options exercised (shares)	(131,666)	(437,906)	(221,987)
Options canceled (in shares)	(17,111)	(42,485)	(153,948)
Option shares outstanding ending balance (in shares)	284,340	433,117	913,508
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Option shares outstanding beginning balance (in dollars per share)	$ 31.51	$ 33	$ 28.41
Options granted (in dollars per shares)			41.09
Options assumed upon acquisition (in dollars per share)			35.03
Options exercised (in dollars per share)	31.75	34.34	25.30
Options canceled (in dollars per share)	34.73	34.26	31.52
Option shares outstanding ending balance (in dollars per share)	31.21	$ 31.51	$ 33
Closing price of company's common stock related to calculation of aggregate intrinsic value of stock options (in dollars per share)	$ 91.36
Stock Option
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Total intrinsic value of options exercised related to stock incentive plans	$ 7.4	$ 9.6	$ 4.7
Aggregate intrinsic value of options outstanding related to stock incentive plans	17.1
Aggregate intrinsic value of options exercisable related to stock incentive plans	$ 13.8
Exercisable number of option shares (in shares)	224,022	264,061
Weighted average exercise price per share of exercisable options (in dollars per share)	$ 29.96	$ 29.59
Weighted average fair value per share of stock options granted			$ 8.85

Equity Incentive Plans Valuatio

Equity Incentive Plans Valuation of Stock Options Granted (Details) - Stock Option	12 Months Ended
	Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 6 months
Volatility	29.50%
Risk-free interest rate	1.54%
Dividend yield	3.00%

Commitments (Details)

Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Future minimum lease commitments under operating leases [Abstract]
2,019	$ 26
2,020	18.7
2,021	13.3
2,022	10.2
2,023	1.7
Thereafter	0
Total minimum payments	69.9
Rental expense under operating lease	30	$ 35.4	$ 23.3
Property, Plant and Equipment Purchase Commitment
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment, remaining minimum amount committed	144.4
Wafer Foundry Purchase Commitment
Purchase Commitment, Excluding Long-term Commitment [Line Items]
Purchase commitment, remaining minimum amount committed	$ 101.6

Geographic and Segment Inform98

Geographic and Segment Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Segment Net Sales	$ 1,002.3	$ 994.2	$ 1,012.1	$ 972.1	$ 902.7	$ 834.4	$ 871.4	$ 799.4	$ 3,980.8	$ 3,407.8	$ 2,173.3
Segment Gross Profit	$ 615.1	$ 607.1	$ 614.1	$ 584.4	$ 532.8	$ 465.3	$ 410.6	$ 348.5	2,420.7	1,757.2	1,205.5
Semiconductor products
Segment Reporting Information [Line Items]
Segment Net Sales									3,876	3,316.6	2,084.2
Segment Gross Profit									2,315.9	1,666	1,116.4
Technology licensing
Segment Reporting Information [Line Items]
Segment Net Sales									104.8	91.2	89.1
Segment Gross Profit									$ 104.8	$ 91.2	$ 89.1

Revenue and Assets by Geographi

Revenue and Assets by Geographic Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Amount of identifiable long-lived assets (consisting of property, plant and equipment)	$ 767.9	$ 683.3
United States
Segment Reporting Information [Line Items]
Amount of identifiable long-lived assets (consisting of property, plant and equipment)	393.3	388.5
Thailand
Segment Reporting Information [Line Items]
Amount of identifiable long-lived assets (consisting of property, plant and equipment)	215.5	178
Various Other Countries besides United States and Thailand
Segment Reporting Information [Line Items]
Amount of identifiable long-lived assets (consisting of property, plant and equipment)	$ 159.1	$ 116.8
Located outside US, in aggregate
Geographic Areas, Revenues from External Customers [Abstract]
Percentage of net sales to unaffiliated customers	85.00%	84.00%	84.00%
Europe
Geographic Areas, Revenues from External Customers [Abstract]
Percentage of net sales to unaffiliated customers	24.00%	24.00%	22.00%
Asia
Geographic Areas, Revenues from External Customers [Abstract]
Percentage of net sales to unaffiliated customers	58.00%	58.00%	59.00%
China, including Hong-Kong
Geographic Areas, Revenues from External Customers [Abstract]
Percentage of net sales to unaffiliated customers	30.00%	32.00%	30.00%
Taiwan
Geographic Areas, Revenues from External Customers [Abstract]
Percentage of net sales to unaffiliated customers	11.00%	9.00%	12.00%

Freestanding Derivative Forward

Freestanding Derivative Forward Contracts (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Percentage Of Company Sales Denominated In Us Dollars	99.00%
Foreign currency transaction gains (loss), before tax	$ 9.3	$ 1	$ 0.7
Foreign Exchange Forward
Derivative [Line Items]
Gain (loss) on foreign currency derivatives	$ 4.2	$ (2.3)

Derivative Instruments Fair Val

Derivative Instruments Fair Value (Details) - Interest Rate Swap - Designated as Hedging Instrument - USD ($) $ in Millions	1 Months Ended
	Feb. 29, 2016	Mar. 31, 2015	Mar. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative, Term of Contract		10 years
Derivative, basis spread on variable rate			53.60%
Derivative, fixed interest rate			1.625%
Derivative asset, notional amount		$ 431.3
Percentage of debt hedged by interest rate derivatives		25.00%
Cash receipts upon termination of interest rate swap agreements	$ 25.7
Derivative, accrued interest received on hedge	$ 3.7

Derivative Instruments Gain (Lo

Derivative Instruments Gain (Loss) (Details) - Interest Rate Swap $ in Millions	12 Months Ended
	Mar. 31, 2016USD ($)
Derivative [Line Items]
Gain (Loss) on 2015 Senior Debt	$ (18.1)
Gain (Loss) on Interest Rate Swap	$ 16.3

Net income per common share 103

Net income per common share attributable to Microchip Technology Stockholders (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended				12 Months Ended
	Nov. 30, 2017	Jun. 30, 2017	Feb. 28, 2017	Feb. 28, 2015	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income from continuing operations attributable to Microchip Technology Stockholders					$ 255.4	$ 170.6	$ 324.1
Weighted average common shares outstanding					232,900,000	217,200,000	203,400,000
Dilutive effect of stock options and RSUs					4,400,000	4,400,000	3,300,000
Weighted average common and potential common shares outstanding					248,900,000	234,800,000	217,400,000
Net income from continuing operations (in usd per share)					$ 1.10	$ 0.79	$ 1.59
Net income from continuing operations (in usd per share)					$ 1.03	$ 0.73	$ 1.49
Number of antidilutive option shares (in shares)					0	0	298,015
2007 Junior Subordinated Convertible Notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures					1,300,000	12,700,000	10,700,000
Shares issued to settle convertible debt					3,700,000
Extinguishment of debt, amount					$ 143.8
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share					$ 23.59	$ 24.01	$ 24.73
2015 Senior Subordinated Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures					10,300,000	500,000	0
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share					$ 63.94	$ 65.21	$ 67.19
2017 Senior Subordinated Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures					0	0	0
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share					$ 99.71	$ 100.58	$ 0
2017 Junior Subordinated Convertible Debt
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of subordinated convertible debentures					0	0	0
Weighted average conversion price per share used in calculating dilutive effect of convertible debt amount per share					$ 97.96	$ 98.81	$ 0
Long-term Debt | 2007 Junior Subordinated Convertible Notes
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Shares issued to settle convertible debt		3,200,000	11,997,924		3,200,000
Extinguishment of debt, amount	$ 14.6	$ 111.3	$ 431.3	$ 575	$ 111.3

Quarterly Results (Unaudited104

Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Net sales	$ 1,002.3	$ 994.2	$ 1,012.1	$ 972.1	$ 902.7	$ 834.4	$ 871.4	$ 799.4	$ 3,980.8	$ 3,407.8	$ 2,173.3
Gross profit	615.1	607.1	614.1	584.4	532.8	465.3	410.6	348.5	2,420.7	1,757.2	1,205.5
Operating income	244.1	245.2	225.4	221.6	154.1	118.1	62.8	(59.1)	936.3	275.8	352.3
Net income from continuing operations	$ 146.7	$ (251.1)	$ 189.2	$ 170.6	$ 136.9	$ 107.2	$ 33.9	$ (113.4)	$ 255.4	$ 164.6	$ 323.9
Net income attributable to Microchip Technology (in usd per share)	$ 0.58	$ (1.07)	$ 0.77	$ 0.70	$ 0.57	$ 0.46	$ 0.14	$ (0.53)	$ 1.03	$ 0.71	$ 1.49
Loss on settlement of convertible debt		$ 2.2		$ 13.8	$ 43.9				$ 16	$ 43.9	$ 0	$ 50.6

Supplemental Financial Infor105

Supplemental Financial Information (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Supplemental Cash Flow Information [Abstract]
Cash paid for income taxes	$ 25.9	$ 48.4	$ 25.4
Cash paid for interest on borrowings	85.3	82.5	52.9
Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balances at Beginning of Year	2.1	2.5	2.6
Charged to Cost and Expense	0.2	0.2	0.1
Deductions	(0.1)	(0.6)	(0.2)
Balance at End of Year	2.2	2.1	2.5
Valuation Allowance of Deferred Tax Assets
Movement in Valuation Allowances and Reserves [Roll Forward]
Balances at Beginning of Year	210.1	161.8	116.5
Charged to Cost and Expense	36.2	15.2	5.5
Charged to Other Accounts	0	37.6	47.8
Deductions	(41.8)	(4.5)	(8)
Balance at End of Year	$ 204.5	$ 210.1	$ 161.8

Comprehensive Income (Schedule

Comprehensive Income (Schedule of Changes in the Components of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Net other comprehensive loss	$ (3.2)	$ (11)	$ (14.1)
Accumulated Net Unrealized Investment Gain (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	0.3	0.3
Other comprehensive loss before reclassifications	(13.6)	(1.5)
Amounts reclassified from accumulated other comprehensive income (loss)	15.2	1.5
Net other comprehensive loss	1.6	0
Stockholders' Equity Attributable to Parent	1.9	0.3	0.3
Accumulated Defined Benefit Plans Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	(5.3)	0
Other comprehensive loss before reclassifications	(5.6)	(5.3)
Amounts reclassified from accumulated other comprehensive income (loss)	0.8	0
Net other comprehensive loss	(4.8)	(5.3)
Stockholders' Equity Attributable to Parent	(10.1)	(5.3)	0
Accumulated Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	(9.4)	(3.7)
Other comprehensive loss before reclassifications	0	(5.7)
Amounts reclassified from accumulated other comprehensive income (loss)	0	0
Net other comprehensive loss	0	(5.7)
Stockholders' Equity Attributable to Parent	(9.4)	(9.4)	(3.7)
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Stockholders' Equity Attributable to Parent	(14.4)	(3.4)
Other comprehensive loss before reclassifications	(19.2)	(12.5)
Amounts reclassified from accumulated other comprehensive income (loss)	16	1.5
Net other comprehensive loss	(3.2)	(11)
Stockholders' Equity Attributable to Parent	$ (17.6)	$ (14.4)	$ (3.4)

Comprehensive Income (Schedu107

Comprehensive Income (Schedule Of Reclassifications Of Recognized Transactions Out Of AOCI) (Details) - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (5.8)	$ 1.3	$ 8.9
Income tax provision	(481.9)	80.8	42.6
Net income attributable to Microchip Technology	255.4	164.6	324.1
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	(15.5)	(1.5)	10.9
Income tax provision	0.3	0	0
Net income attributable to Microchip Technology	(16)	(1.5)	10.9
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other (loss) income, net	$ (0.8)	$ 0	$ 0

Dividends (Details)

Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Dividends [Abstract]
Total dividends per share for the period (in USD per share)	$ 1.449	$ 1.441	$ 1.433
Total dividend payments	$ 337.5	$ 315.4	$ 291.1

Subsequent Events Business Acqu

Subsequent Events Business Acquisitions (Narrative) (Details)	May 18, 2018USD ($)	Mar. 31, 2018USD ($)employee	Mar. 31, 2018USD ($)employee	Jun. 30, 2018USD ($)	Mar. 01, 2018USD ($)employee$ / shares	Jun. 30, 2017USD ($)
Microsemi
Subsequent Event [Line Items]
Business acquisition, share price (in usd per share) | $ / shares					$ 68.78
Bridge Loan | Microsemi
Subsequent Event [Line Items]
Long term debt (up to)					$ 600,000,000
Bridge Loan | Term Loan Facility | Microsemi
Subsequent Event [Line Items]
Long term debt (up to)					$ 5,000,000,000
Debt instrument, term (in days)		364 days
Revolving Credit Facility
Subsequent Event [Line Items]
Maximum borrowing capacity						$ 2,774,000,000
Revolving Credit Facility | 2020 Tranche
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 3,122,000,000	$ 3,122,000,000
Foreign Line of Credit
Subsequent Event [Line Items]
Maximum borrowing capacity		125,000,000	125,000,000
Standby Letters of Credit
Subsequent Event [Line Items]
Maximum borrowing capacity		25,000,000	25,000,000
Line of Credit Facility Swingline Loan Sublimit
Subsequent Event [Line Items]
Maximum borrowing capacity		$ 25,000,000	25,000,000
Building
Subsequent Event [Line Items]
Purchase of building			$ 40,800,000
Microsemi
Subsequent Event [Line Items]
Number of employees | employee		4,800	4,800		4,800
Scenario, Forecast | Microsemi
Subsequent Event [Line Items]
Agreement termination fee				$ 290,000,000
Contingent reimbursement fee				35,000,000
Equity value acquired				8,350,000,000
Enterprise value				$ 10,150,000,000
Subsequent Event | Revolving Credit Facility
Subsequent Event [Line Items]
Maximum borrowing capacity	$ 3,800,000,000
Subsequent Event | Revolving Credit Facility | 2023 Tranche
Subsequent Event [Line Items]
Maximum borrowing capacity	3,600,000,000
Subsequent Event | Revolving Credit Facility | 2020 Tranche
Subsequent Event [Line Items]
Maximum borrowing capacity	244,000,000
Subsequent Event | Foreign Line of Credit
Subsequent Event [Line Items]
Maximum borrowing capacity	250,000,000
Subsequent Event | Standby Letters of Credit
Subsequent Event [Line Items]
Maximum borrowing capacity	50,000,000
Subsequent Event | Line of Credit Facility Swingline Loan Sublimit
Subsequent Event [Line Items]
Maximum borrowing capacity	$ 25,000,000
Subsequent Event | Base Rate | Minimum | Revolving Credit Facility | 2023 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	0.00%
Subsequent Event | Base Rate | Minimum | Revolving Credit Facility | 2020 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	0.25%
Subsequent Event | Base Rate | Maximum | Revolving Credit Facility | 2023 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	1.00%
Subsequent Event | Base Rate | Maximum | Revolving Credit Facility | 2020 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	1.25%
Subsequent Event | London Interbank Offered Rate (LIBOR) | Minimum | Revolving Credit Facility | 2023 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	1.00%
Subsequent Event | London Interbank Offered Rate (LIBOR) | Minimum | Revolving Credit Facility | 2020 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	1.25%
Subsequent Event | London Interbank Offered Rate (LIBOR) | Maximum | Revolving Credit Facility | 2023 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	2.00%
Subsequent Event | London Interbank Offered Rate (LIBOR) | Maximum | Revolving Credit Facility | 2020 Tranche
Subsequent Event [Line Items]
Basis spread on variable rate	2.25%