Financial Instruments (Details) - Measured on a recurring basis - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Fair Value Measurements
Adjusted Cost, assets	$ 83,199	$ 85,918
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities	(97)	(62)
Fair Value, assets	83,103	85,856
Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	27,229	29,761
Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	53,317	44,057
Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	2,557	12,038
Cash
Fair Value Measurements
Fair value, cash and cash equivalents	21,145	24,367
Cash \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	21,145	24,367
Level 1
Fair Value Measurements
Adjusted Cost, assets	14,022	14,454
Unrealized Losses, available-for-sale securities	(32)	(13)
Fair Value, assets	13,990	14,441
Level 1 \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	5,085	5,394
Level 1 \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	7,430	4,098
Level 1 \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,475	4,949
Level 1 \| Money market funds
Fair Value Measurements
Fair value, cash and cash equivalents	5,085	5,394
Level 1 \| Money market funds \| Cash and Cash Equivalents
Fair Value Measurements
Fair value, cash and cash equivalents	5,085	5,394
Level 1 \| U.S. government notes
Fair Value Measurements
Adjusted Cost, available-for-sale securities	8,937	9,060
Unrealized Losses, available-for-sale securities	(32)	(13)
Fair Value, available-for-sale investments	8,905	9,047
Level 1 \| U.S. government notes \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	7,430	4,098
Level 1 \| U.S. government notes \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,475	4,949
Level 2 \| Available-for-sale investments
Fair Value Measurements
Adjusted Cost, available-for-sale securities	48,032	47,097
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities	(65)	(49)
Fair Value, available-for-sale investments	47,968	47,048
Level 2 \| Available-for-sale investments \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	999
Level 2 \| Available-for-sale investments \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	45,887	39,959
Level 2 \| Available-for-sale investments \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,082	7,089
Level 2 \| Corporate bonds
Fair Value Measurements
Adjusted Cost, available-for-sale securities	26,951	24,943
Unrealized Gains, available-for-sale securities	1
Unrealized Losses, available-for-sale securities	(65)	(49)
Fair Value, available-for-sale investments	26,887	24,894
Level 2 \| Corporate bonds \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	25,805	17,805
Level 2 \| Corporate bonds \| Long-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	1,082	7,089
Level 2 \| Commercial paper
Fair Value Measurements
Adjusted Cost, available-for-sale securities	21,081	22,154
Fair Value, available-for-sale investments	21,081	22,154
Level 2 \| Commercial paper \| Cash and Cash Equivalents
Fair Value Measurements
Fair Value, available-for-sale investments	999
Level 2 \| Commercial paper \| Short-term investments
Fair Value Measurements
Fair Value, available-for-sale investments	$ 20,082	$ 22,154

Represents the adjusted cost of cash and cash equivalents and investments.

No definition available.

Name:	ctrl_CashAndCashEquivalentsAndInvestmentsAdjustedCost
Namespace Prefix:	ctrl_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

+ Details

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

- Definition

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

+ References

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=75030427&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=82897076&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=82896770&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=82897076&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=82897076&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

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=ctrl_CashAndCashEquivalentsBalanceSheetLocationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=ctrl_ShortTermInvestmentsBalanceSheetLocationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=ctrl_LongTermInvestmentsBalanceSheetLocationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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