Basis of Presentation - Additional Information (Detail)		3 Months Ended			6 Months Ended		12 Months Ended
	Jul. 23, 2020 USD ($) $ / shares shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2018 USD ($)	Sep. 30, 2020 USD ($) Segment shares	Sep. 30, 2019 USD ($) shares	Mar. 31, 2020 USD ($) shares	Mar. 31, 2019 USD ($) shares	Jun. 30, 2020 shares	Jun. 30, 2019 shares
Company And Summary Of Significant Accounting Policies [Line Items]
Net proceeds form sale of shares after deducting offering expenes	$ 174,700,000				$ 174,749,000
Revenue, practical expedient, financing component					true
Remaining performance obligations		$ 2,300,000,000			$ 2,300,000,000
Number of reportable segments | Segment					3
Decrease in unbilled accounts receivable					$ 3,500,000
Increase in collections in excess of revenues and deferred revenues					46,600,000
Collections in excess of revenues and deferred revenues, recognized revenue		17,000,000.0	$ 18,600,000		74,600,000	$ 71,200,000
Capitalized interest expense		20,000,000.0	13,000,000.0		35,900,000	24,300,000
Property, equipment and satellites		4,498,958,000			4,498,958,000		$ 4,101,634,000
Accumulated depreciation and amortization		1,650,318,000			1,650,318,000		1,514,899,000
Proceeds from insurance claims on ViaSat-2 satellite						2,277,000	188,000,000.0	$ 188,000,000.0
Total capitalized costs related to patents		3,400,000			3,400,000		3,300,000
Total capitalized costs related to orbital slots and other licenses		48,200,000			48,200,000		39,500,000
Accumulated amortization of patents, orbital slots and other licenses		4,000,000.0			4,000,000.0		3,700,000
Debt issuance costs capitalized					5,100,000	0
Capitalized costs, net, related to software developed for resale		243,195,000			243,195,000		242,741,000
Capitalized cost related to software development for resale		14,200,000	13,900,000		28,500,000	23,600,000
Amortization expense of capitalized software development costs		14,900,000	13,700,000		28,000,000.0	25,700,000
Self-insurance liability		6,800,000			6,800,000		6,200,000
Repurchase and immediate retirement of treasury shares pursuant to vesting of certain RSU agreements		346,000	800,000		747,000	3,128,000
Stock-based compensation expense		22,400,000	$ 21,100,000		$ 43,328,000	$ 42,328,000
Accounting Standards Update 2016-13 [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Description of new accounting pronouncements					In June 2016, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) 2016-13, Financial Instruments — Credit Losses (ASC 326). ASU 2016-13 requires credit losses on most financial assets measured at amortized cost and certain other instruments to be measured using an expected credit loss model (referred to as the current expected credit loss model). It also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. In November 2018, the FASB issued ASU 2018-19, Codification Improvements to ASC 326, Financial Instruments — Credit Losses (ASC 326), which clarifies that impairment of receivables arising from operating leases should be accounted for in accordance with ASC 842, Leases. In April 2019, the FASB issued ASU 2019-04, Codification Improvements to ASC 326, Financial Instruments — Credit Losses, in May 2019, the FASB issued ASU 2019-05, Financial Instruments — Credit Losses (ASC 326) Targeted Relief, in November 2019, the FASB issued ASU 2019-11, Codification Improvements to ASC 326, Financial Instruments — Credit Losses, in February 2020, the FASB issued ASU 2020-02, Financial Instruments — Credit Losses (ASC 326) and Leases (ASC 842) Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 119 and Update to SEC Section on Effective Date Related to ASU 2016-02, Leases (ASC 842) and in March 2020, the FASB issued ASU 2020-03, Codification Improvements to Financial Instruments. These recently issued ASUs do not change the core principle of the guidance in ASU 2016-13 but rather are intended to clarify and improve operability of certain topics included within ASU 2016-13. ASU 2018-19, ASU 2019-04, ASU 2019-05, ASU 2019-11, ASU 2020-02, and ASU 2020-03 have the same effective date and transition requirements as ASU 2016-13. The Company adopted the new guidance in the first quarter of fiscal year 2021 using the modified retrospective approach with application of the model to the Company’s accounts receivables. Under the new standard, the Company is required to recognize estimated credit losses expected to occur over the estimated life or remaining contractual life of an asset using a broader range of information including past events, current conditions and consideration of supportable forecasts about future economic conditions. The adoption of the standard had an insignificant impact on the Company’s consolidated financial statements and disclosures.
Accounting Standards Update 2018-13 [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Description of new accounting pronouncements					In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (ASC 820): Disclosure Framework—Changes to the Disclosure Requirements for Fair Value Measurement, which modifies the disclosure requirements for fair value measurements by removing, modifying, or adding certain disclosures. The Company adopted the new guidance in the first quarter of fiscal year 2021 and the guidance did not have a material impact on the Company’s consolidated financial statements and disclosures.
Accounting Standards Update 2019-12 [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Description of new accounting pronouncements					In December 2019, the FASB issued ASU 2019-12, Income Taxes (ASC 740): Simplifying the Accounting for Income Taxes, which is intended to simplify various areas related to accounting for income taxes. ASU 2019-12 removes certain exceptions to the general principles in ASC 740 and also clarifies and amends existing guidance to improve consistent application. ASU 2019-12 is effective for the Company beginning in fiscal year 2022, with early adoption permitted. The Company is currently evaluating the impact of this standard on its consolidated financial statements and disclosures.
Accounting Standards Update 2020-01 [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Description of new accounting pronouncements					In January 2020, the FASB issued ASU 2020-01, Investments – Equity Securities (ASC 321), Investments – Equity Method and Joint Ventures (ASC 323) and Derivatives and Hedging (ASC 815). ASU 2020-01 clarifies the interaction of the accounting for equity securities under ASC 321 and investments accounted for under the equity method of accounting under ASC 323, and the accounting for certain forward contracts and purchased options accounted for under ASC 815. The new standard will become effective for the Company beginning in fiscal year 2022, with early adoption permitted. The Company is currently evaluating the impact of this standard on its consolidated financial statements and disclosures.
Accounting Standards Update 2020-04 [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Description of new accounting pronouncements					In March 2020, the FASB issued ASU 2020-04, Reference Rate Reform (ASC 848): Facilitation of the Effects of Reference Rate Reform on Financial Reporting. ASU 2020-04 provides temporary optional guidance to ease the potential accounting burden associated with the transition away from reference rates (such as the London Interbank Offered Rate) that are expected to be discontinued. ASU 2020-04 was effective upon issuance and can be applied for a limited time through December 31, 2022. The Company adopted the guidance upon issuance with no impact to the Company consolidated financial statements and disclosures.
Accounting Standards Update 2020-06 [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Description of new accounting pronouncements					In August 2020, the FASB issued ASU 2020-06, Debt – Debt with Conversion and Other Options (ASC 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40). ASU 2020-06 simplifies the accounting for convertible instruments by removing the beneficial conversion and cash conversion accounting models for convertible instruments and removes certain settlement conditions that are required for contracts to qualify for equity classification. This new standard also simplifies the diluted earnings per share calculations by requiring that an entity use the if-converted method for convertible instruments and requires that the effect of potential share settlement be included in diluted earnings per share calculations when an instrument may be settled in cash or shares. The new standard requires entities to provide expanded disclosures about the terms and features of convertible instruments, how the instruments have been reported in the entity’s financial statements, and information about events, conditions, and circumstances that can affect how to assess the amount or timing of an entity’s future cash flows related to those instruments. The new standard will become effective for the Company beginning in fiscal year 2023, with early adoption permitted, but no earlier than fiscal year 2022. The Company is currently evaluating the impact of this standard on its consolidated financial statements and disclosures.
CPE Leased Equipment [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites		408,732,000			$ 408,732,000		399,343,000
Accumulated depreciation and amortization		$ 179,100,000			$ 179,100,000		165,700,000
ViaSat-2 Satellite [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Reduction in property and equipment, net				$ 177,400,000
Estimated insurance claim receivable				$ 177,400,000
Proceeds from insurance claims on ViaSat-2 satellite							188,000,000.0	$ 188,000,000.0
Minimum [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites, estimated useful life (years)					2 years
Estimated useful life, years					2 years
Minimum [Member] | Internally Developed Software [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites, estimated useful life (years)					3 years
Minimum [Member] | CPE Leased Equipment [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites, estimated useful life (years)					4 years
Maximum [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites, estimated useful life (years)					17 years
Estimated useful life, years					10 years
Maximum [Member] | Software Development Costs [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Estimated useful life, years					5 years
Maximum [Member] | Internally Developed Software [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites, estimated useful life (years)					7 years
Maximum [Member] | CPE Leased Equipment [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Property, equipment and satellites, estimated useful life (years)					5 years
Funded Research and Development from Customer Contracts [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenue		23.00%	22.00%		25.00%	23.00%
Operating Segments [Member] | Commercial Networks and Government Systems [Member] | Fixed-price Contract [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenue		87.00%	89.00%		86.00%	88.00%
U.S. Government as an Individual Customer [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenue		32.00%	30.00%		31.00%	30.00%
Commercial Customers [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Percentage of revenue		68.00%	70.00%		69.00%	70.00%
Unfavorable Regulatory Action [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Accrued reserves		$ 8,300,000			$ 8,300,000		7,800,000
Indemnification Agreement [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Accrued reserves		$ 0			$ 0		$ 0
Common Stock [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Number of shares issued and sold | shares	4,474,559	67,532,869	61,352,560		67,532,869	61,352,560	62,147,140	60,550,093	63,026,001	61,276,933
Shares sold, purchase price | $ / shares	$ 39.11
Common stock issued based on the vesting terms of certain restricted stock unit agreements | shares		40,676	43,290		64,970	115,483
Common Stock Held in Treasury [Member]
Company And Summary Of Significant Accounting Policies [Line Items]
Shares of common stock outstanding | shares		0			0		0
Purchase of treasury shares pursuant to vesting of certain RSU agreements | shares		8,367	10,269		16,915	36,132
Repurchase and immediate retirement of treasury shares pursuant to vesting of certain RSU agreements						$ 3,100,000