Investments - Gross Unrealized Gains and (Losses) Aggregated by Investment Category Short-Term and Long-Term Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Short-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	$ 177,523	$ 129,600
Investments, Gross Unrealized Gains	39	16
Investments, Gross Unrealized (Losses)	(129)	(22)
Investments, Estimated Fair Value	177,433	129,594
Short-Term Investments [Member] \| Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	23,630	20,901
Investments, Gross Unrealized (Losses)		(1)
Investments, Estimated Fair Value	23,630	20,900
Short-Term Investments [Member] \| Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	256
Investments, Gross Unrealized (Losses)	(101)
Investments, Estimated Fair Value	155
Short-Term Investments [Member] \| Bankers' Acceptance Drafts [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	222	82
Investments, Estimated Fair Value	222	82
Short-Term Investments [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	3,449
Investments, Gross Unrealized Gains	1
Investments, Estimated Fair Value	3,450
Short-Term Investments [Member] \| Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	59,735	24,029
Investments, Gross Unrealized Gains	7	2
Investments, Gross Unrealized (Losses)	(23)	(11)
Investments, Estimated Fair Value	59,719	24,020
Short-Term Investments [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	7,055	2,100
Investments, Gross Unrealized Gains	6
Investments, Gross Unrealized (Losses)	(4)	(1)
Investments, Estimated Fair Value	7,057	2,099
Short-Term Investments [Member] \| U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	83,176	82,488
Investments, Gross Unrealized Gains	25	14
Investments, Gross Unrealized (Losses)	(1)	(9)
Investments, Estimated Fair Value	83,200	82,493
Long-Term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	257,017	157,489
Investments, Gross Unrealized Gains	73	8
Investments, Gross Unrealized (Losses)	(235)	(296)
Investments, Estimated Fair Value	256,855	157,201
Long-Term Investments [Member] \| Time Deposits and Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost		48
Investments, Estimated Fair Value		48
Long-Term Investments [Member] \| Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	140,695	75,778
Investments, Gross Unrealized Gains	50	5
Investments, Gross Unrealized (Losses)	(68)	(109)
Investments, Estimated Fair Value	140,677	75,674
Long-Term Investments [Member] \| Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	103,709	64,842
Investments, Gross Unrealized Gains	17	1
Investments, Gross Unrealized (Losses)	(156)	(174)
Investments, Estimated Fair Value	103,570	64,669
Long-Term Investments [Member] \| Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	615	1,258
Investments, Gross Unrealized (Losses)	(10)	(4)
Investments, Estimated Fair Value	605	1,254
Long-Term Investments [Member] \| U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Investments, Cost	11,998	15,563
Investments, Gross Unrealized Gains	6	2
Investments, Gross Unrealized (Losses)	(1)	(9)
Investments, Estimated Fair Value	$ 12,003	$ 15,556

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 5

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=mksi_TimeDepositsAndCertificatesOfDepositMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_BankersAcceptanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CommercialPaperMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis=us-gaap_OtherLongTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: