CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)		3 Months Ended				6 Months Ended		12 Months Ended
		Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2019
Cash flows from operating activities:
Net income		$ 108,900,000	$ 50,700,000	$ 96,300,000	$ 35,700,000	$ 159,600,000	$ 132,000,000.0
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization						604,700,000	393,800,000
Deferred income taxes						(110,900,000)	(71,600,000)
Share-based compensation expense related to equity incentive plans						86,000,000.0	87,900,000
Loss on settlement of debt		100,000		4,100,000		2,000,000.0	4,100,000
Amortization of debt discount						60,200,000	56,300,000
Amortization of debt issuance costs						8,500,000	7,500,000
Losses on equity method investments		0		100,000		0	100,000
Impairment of intangible assets		0		1,200,000		500,000	3,100,000
(Gains) losses on available-for-sale investments and marketable equity securities, net						(1,000,000.0)	6,200,000
Amortization of premium on available-for-sale investments						0	(200,000)
Changes in operating assets and liabilities, excluding impact of acquisitions:
Decrease in accounts receivable						32,100,000	65,400,000
(Increase) decrease in inventories						(20,200,000)	261,500,000
Decrease in accounts payable and accrued liabilities						(8,400,000)	(37,800,000)
Change in other assets and liabilities						(900,000)	(25,100,000)
Change in income tax payable						(35,600,000)	(93,300,000)
Net cash provided by operating activities						776,600,000	789,900,000
Cash flows from investing activities:
Purchases of available-for-sale investments						0	(167,700,000)
Maturities of available-for-sale investments						0	75,900,000
Sales of available-for-sale investments						0	1,376,600,000	$ 1,380,000,000
Acquisition of Microsemi, net of cash acquired						0	(7,850,600,000)
Investments in other assets						(13,600,000)	(8,900,000)
Proceeds from sale of assets						500,000	100,000
Capital expenditures						(41,600,000)	(161,400,000)
Net cash used in investing activities						(54,700,000)	(6,736,000,000.0)
Cash flows from financing activities:
Proceeds from issuance of 2023 and 2021 Senior Notes						0	1,989,500,000
Repayment of debt assumed in Microsemi acquisition						0	(2,056,900,000)
Deferred financing costs						(1,800,000)	(72,700,000)
Payment of cash dividends						(174,400,000)	(171,400,000)
Proceeds from sale of common stock						27,500,000	20,300,000
Tax payments related to shares withheld for vested restricted stock units						(26,600,000)	(36,900,000)
Capital lease payments						(400,000)	(400,000)
Net cash (used in) provided by financing activities						(748,700,000)	5,504,500,000
Net decrease in cash and cash equivalents						(26,800,000)	(441,600,000)
Cash and cash equivalents, and restricted cash at beginning of period	[1]		$ 428,600,000		$ 901,300,000	428,600,000	901,300,000	901,300,000
Cash and cash equivalents, and restricted cash at end of period		401,800,000		$ 459,700,000		401,800,000	459,700,000	428,600,000	[1]
Non-cash activities:
Right-of-use assets obtained in exchange of lease liabilities						17,300,000
Cash paid for:
Operating lease payments in operating cash flows						13,100,000
Restricted Cash		$ 37,300,000				37,300,000		$ 38,400,000
Term Loan Facility
Cash flows from financing activities:
Proceeds from borrowings under credit facility						0	3,000,000,000.0
Repayments of credit facility						(188,000,000.0)	(267,000,000.0)
Revolving Credit Facility
Cash flows from financing activities:
Proceeds from borrowings under credit facility						427,000,000.0	3,632,500,000
Repayments of credit facility						$ (812,000,000.0)	$ (532,500,000)

[1] Schedule of restricted cash
The following table presents the balance of restricted cash which consists of cash denominated in a foreign currency and restricted in use due to a foreign taxing authority requirement (in millions):
  September 30,   March 31,   2019   2019 Restricted cash $ 37.3
  $ 38.4