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Summary of Significant Accounting Policies (Details) (USD $)
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Payment
Dec. 31, 2013
Dec. 31, 2012
Home office and equipment [Abstract]                      
Accumulated depreciation of home office and equipment $ 54,900,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment       $ 53,000,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment       $ 54,900,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment $ 53,000,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment $ 51,300,000us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Depreciation expense of home office and equipment                 2,200,000us-gaap_Depreciation 1,800,000us-gaap_Depreciation 1,900,000us-gaap_Depreciation
Loss Reserves [Abstract]                      
Minimum number of payments past due to be in default                 2mtg_MinimumNumberOfPaymentsPastDueToBeInDefault    
Share-Based Compensation [Abstract]                      
Minimum vesting period for share-based compensation awards (in years)                 1 year    
Maximum vesting period for share-based compensation awards (in years)                 3 years    
Basic earnings (loss) per share [Abstract]                      
Net income (loss) 74,428,000us-gaap_NetIncomeLoss 72,017,000us-gaap_NetIncomeLoss 45,522,000us-gaap_NetIncomeLoss 59,982,000us-gaap_NetIncomeLoss (1,407,000)us-gaap_NetIncomeLoss 12,114,000us-gaap_NetIncomeLoss 12,375,000us-gaap_NetIncomeLoss (72,930,000)us-gaap_NetIncomeLoss 251,949,000us-gaap_NetIncomeLoss (49,848,000)us-gaap_NetIncomeLoss (927,079,000)us-gaap_NetIncomeLoss
Average common shares outstanding (in shares)                 338,523,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 311,754,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 201,892,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Basic income (loss) per share (in dollars per share) $ 0.22us-gaap_EarningsPerShareBasic [1],[2] $ 0.21us-gaap_EarningsPerShareBasic [1],[2] $ 0.13us-gaap_EarningsPerShareBasic [1],[2] $ 0.18us-gaap_EarningsPerShareBasic [1],[2] $ 0us-gaap_EarningsPerShareBasic [1] $ 0.04us-gaap_EarningsPerShareBasic [1] $ 0.04us-gaap_EarningsPerShareBasic [1] $ (0.31)us-gaap_EarningsPerShareBasic [1] $ 0.74us-gaap_EarningsPerShareBasic $ (0.16)us-gaap_EarningsPerShareBasic $ (4.59)us-gaap_EarningsPerShareBasic
Diluted earnings (loss) per share [Abstract]                      
Net income (loss) 74,428,000us-gaap_NetIncomeLoss 72,017,000us-gaap_NetIncomeLoss 45,522,000us-gaap_NetIncomeLoss 59,982,000us-gaap_NetIncomeLoss (1,407,000)us-gaap_NetIncomeLoss 12,114,000us-gaap_NetIncomeLoss 12,375,000us-gaap_NetIncomeLoss (72,930,000)us-gaap_NetIncomeLoss 251,949,000us-gaap_NetIncomeLoss (49,848,000)us-gaap_NetIncomeLoss (927,079,000)us-gaap_NetIncomeLoss
Diluted income available to common shareholders                 264,146,000us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted (49,848,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted (927,079,000)us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted
Weighted-average shares - Basic (in shares)                 338,523,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 311,754,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic 201,892,000us-gaap_WeightedAverageNumberOfSharesOutstandingBasic
Effect of dilutive securities [Abstract]                      
2% Convertible Senior Notes due 2020                 $ 12,197,000us-gaap_DilutiveSecurities $ 0us-gaap_DilutiveSecurities $ 0us-gaap_DilutiveSecurities
Unvested restricted stock (in shares)                 3,082,000us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 0us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements 0us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements
Convertible debt common stock equivalents (in shares)                 71,942,000us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment 0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment 0us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustment
Weighted-average shares - Diluted (in shares)                 413,547,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 311,754,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding 201,892,000us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding
Diluted income (loss) per share (in dollars per share) $ 0.19us-gaap_EarningsPerShareDiluted [1],[2] $ 0.18us-gaap_EarningsPerShareDiluted [1],[2] $ 0.12us-gaap_EarningsPerShareDiluted [1],[2] $ 0.15us-gaap_EarningsPerShareDiluted [1],[2] $ 0us-gaap_EarningsPerShareDiluted [1] $ 0.04us-gaap_EarningsPerShareDiluted [1] $ 0.04us-gaap_EarningsPerShareDiluted [1] $ (0.31)us-gaap_EarningsPerShareDiluted [1] $ 0.64us-gaap_EarningsPerShareDiluted $ (0.16)us-gaap_EarningsPerShareDiluted $ (4.59)us-gaap_EarningsPerShareDiluted
Convertible senior notes percentage (in hundredths) 2.00%us-gaap_DebtInstrumentInterestRateStatedPercentage               2.00%us-gaap_DebtInstrumentInterestRateStatedPercentage    
Participating Securities [Member]                      
Earnings per Share [Abstract]                      
Participating securities included weighted average number of common shares outstanding (in shares)                 100,000mtg_ParticipatingSecuritiesIncludedWeightedAverageNumberOfCommonSharesOutstanding
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= mtg_ParticipatingSecuritiesMember		    
Antidilutive securities excluded from weighted average number of shares (in shares)                   100,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= mtg_ParticipatingSecuritiesMember		 1,100,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= mtg_ParticipatingSecuritiesMember
Convertible Debt Securities [Member]                      
Earnings per Share [Abstract]                      
Antidilutive securities excluded from weighted average number of shares (in shares)                 54,500,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember		 126,400,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember		 60,700,000us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_ConvertibleDebtSecuritiesMember
Restricted Stock Units (RSUs) [Member]                      
Earnings per Share [Abstract]                      
Participating securities included weighted average number of common shares outstanding (in shares)                 3,100,000mtg_ParticipatingSecuritiesIncludedWeightedAverageNumberOfCommonSharesOutstanding
/ us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis
= us-gaap_RestrictedStockUnitsRSUMember		    
Home Office [Member]                      
Home office and equipment [Abstract]                      
Estimated useful life                 45 years    
Equipment [Member]                      
Home office and equipment [Abstract]                      
Estimated useful life                 5 years    
Data Processing Hardware [Member]                      
Home office and equipment [Abstract]                      
Estimated useful life                 3 years    
[1] Due to the use of weighted average shares outstanding when calculating earnings per share, the sum of the quarterly per share data may not equal the per share data for the year.
[2] In periods where convertible debt instruments are dilutive to earnings per share the "if-converted" method of computing diluted EPS requires an interest expense adjustment, net of tax, to net income available to shareholders. This adjustment has not been reflected in the Unaudited Quarterly Financial Data presented. See Note 3 - "Summary of Significant Accounting Policies" for further discussion.