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v2.4.1.9

Summary of Significant Accounting and Reporting Policies (Schedule of Amortized Cost, Unrealized Gains And Losses, And Market Value Of Investment Securities) (Details) (USD $) | Dec. 31, 2014 | Dec. 31, 2013
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Schedule of Available-for-sale Securities [Line Items] |  |
Cost | $ 97,923,092 us-gaap_AvailableForSaleSecuritiesAmortizedCost | $ 77,055,612 us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains | 17,855,921 gntx_AvailableForSaleSecuritiesGrossUnrealizedGainsAccumulatedInAOCI | 29,998,513 gntx_AvailableForSaleSecuritiesGrossUnrealizedGainsAccumulatedInAOCI
Gross Unrealized Losses | (1,135,425)gntx_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInAOCI | (48,603)gntx_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInAOCI
Market Value | 114,643,588 us-gaap_InvestmentsFairValueDisclosure | 107,005,522 us-gaap_InvestmentsFairValueDisclosure
Other |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | 1,021 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |
Market Value | 1,021 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_OtherDebtSecuritiesMember |
Common Stocks |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | 17,069,742 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 22,799,035 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Gross Unrealized Gains | 7,933,717 gntx_AvailableForSaleSecuritiesGrossUnrealizedGainsAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 10,532,007 gntx_AvailableForSaleSecuritiesGrossUnrealizedGainsAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Gross Unrealized Losses | (355,008)gntx_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | (48,603)gntx_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Market Value | 24,648,451 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember | 33,282,439 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CommonStockMember
Mutual Funds – Equity |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Cost | 80,852,329 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember | 54,256,577 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember
Gross Unrealized Gains | 9,922,204 gntx_AvailableForSaleSecuritiesGrossUnrealizedGainsAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember | 19,466,506 gntx_AvailableForSaleSecuritiesGrossUnrealizedGainsAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember
Gross Unrealized Losses | (780,417)gntx_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember | 0 gntx_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInAOCI / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember
Market Value | $ 89,994,116 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember | $ 73,723,083 us-gaap_InvestmentsFairValueDisclosure / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = gntx_MutualFundsMember

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