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Securities - Additional Information (Detail) (USD $) | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2012
Schedule Of Marketable Securities [Line Items] |  |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas | 97.50%cfr_PercentMunicipalBondPortfolioIssuedByPoliticalSubdivisionOrAgenciesWithinStateOfTexas |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 64.10%cfr_PercentMunicipalBondPortfolioIssuedByPoliticalSubdivisionOrAgenciesWithinStateOfTexasGuaranteedByTexasPermanentSchoolFund |  |
Securities Pledged as Collateral | $ 3,100,000,000 us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral | $ 3,000,000,000 us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral |
Available for Sale, Estimated Fair Value | 8,672,497,000 us-gaap_AvailableForSaleSecurities | 8,461,254,000 us-gaap_AvailableForSaleSecurities |
Available for sale securities reclassified to held to maturity, unrealized gain |  |  | 165,700,000 cfr_AvailableForSaleSecuritiesTransfersToHeldToMaturityUnrealizedGains
Available for sale securities reclassified to held to maturity, unrealized gain net of tax |  |  | 107,700,000 cfr_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesUnrealizedGainsNetOfTax
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax | 86,000,000 cfr_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentPreTax |  |
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 55,900,000 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax |  |
Reclassified To Held To Maturity [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Available for Sale, Estimated Fair Value |  |  | $ 2,300,000,000 us-gaap_AvailableForSaleSecurities / us-gaap_InformationByCategoryOfDebtSecurityAxis = cfr_ReclassifiedToHeldToMaturityMember

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- Definition Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas + References No definition available. + Details
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- Definition Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas Guaranteed By Texas Permanent School Fund + References No definition available. + Details
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- Definition Accumulated appreciation or loss, net of tax, in value of the total of available-for-sale securities at the end of an accounting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 + Details
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- Definition The fair value of available-for-sale securities that serve as collateral for borrowings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=49124121&loc=d3e5086-111524 + Details
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