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Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
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Fair Value of Financial Instruments |  |  |  |
Cash and cash equivalents, cost | $ 3,110 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 4,364 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 2,529 us-gaap_CashAndCashEquivalentsAtCarryingValue | $ 40,025 us-gaap_CashAndCashEquivalentsAtCarryingValue
Cash and cash equivalents, fair value | 3,110 us-gaap_CashAndCashEquivalentsFairValueDisclosure | 4,364 us-gaap_CashAndCashEquivalentsFairValueDisclosure |  |
Estimated fair values of financial instruments |  |  |  |
Unrealized Gains | 3 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax | 23 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax |  |
Unrealized Losses | (13)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax | (10)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax |  |
Short-term investments. |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 20,442 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 32,183 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  |
Unrealized Gains | 3 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 14 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  |
Unrealized Losses | (6)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | (5)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  |
Fair Value | 20,439 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 32,192 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember |  |
Short-term investments. | U.S. government-sponsored enterprise bonds |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 1,250 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember | 6,543 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember |  |
Unrealized Losses |  | (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember |  |
Fair Value | 1,250 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember | 6,542 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember |  |
Short-term investments. | Municipal Bonds |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 841 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 13,401 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Unrealized Gains |  | 11 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Fair Value | 841 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember | 13,412 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Short-term investments. | Corporate notes |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 15,921 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 7,245 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Unrealized Gains | 2 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 2 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Unrealized Losses | (6)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | (2)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Fair Value | 15,917 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 7,245 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Short-term investments. | Certificates of deposit |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 2,430 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember | 4,994 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |
Unrealized Gains | 1 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember | 1 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |
Unrealized Losses |  | (2)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |
Fair Value | 2,431 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember | 4,993 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |
Long-term investments |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 2,252 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember | 13,922 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember |  |
Unrealized Gains |  | 9 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember |  |
Unrealized Losses | (7)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember | (5)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember |  |
Fair Value | 2,245 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember | 13,926 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember |  |
Long-term investments | U.S. government-sponsored enterprise bonds |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 1,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember | 2,752 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember |  |
Unrealized Losses | (2)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember |  |  |
Fair Value | 998 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember | 2,752 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentDebtSecuritiesMember |  |
Long-term investments | Municipal Bonds |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost |  | 264 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Fair Value |  | 264 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MunicipalBondsMember |  |
Long-term investments | Corporate notes |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost | 1,252 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 9,466 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Unrealized Gains |  | 9 us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Unrealized Losses | (5)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Fair Value | 1,247 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 9,474 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember |  |
Long-term investments | Certificates of deposit |  |  |  |
Estimated fair values of financial instruments |  |  |  |
Cost |  | 1,440 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |
Unrealized Losses |  | (4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |
Fair Value |  | $ 1,436 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = mosy_LongTermInvestmentsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CertificatesOfDepositMember |  |

X
- 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=28360136&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=6871852&loc=d3e26610-111562 + Details
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- 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
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- 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
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- 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
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- Definition Amount 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. Excludes cash and cash equivalents within disposal group and discontinued operation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3044-108585 + Details
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