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Securities (Narrative) (Detail) (USD $) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | 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 | 97.40%cfr_PercentMunicipalBondPortfolioIssuedByPoliticalSubdivisionOrAgenciesWithinStateOfTexas |  |
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund | 64.90%cfr_PercentMunicipalBondPortfolioIssuedByPoliticalSubdivisionOrAgenciesWithinStateOfTexasGuaranteedByTexasPermanentSchoolFund |  |
Carrying value of pledged securities | $ 3,000,000,000 cfr_CarryingValueOfPledgedSecurities | $ 3,000,000,000 cfr_CarryingValueOfPledgedSecurities |
Securities available for sale, at estimated fair value | 8,461,254,000 us-gaap_AvailableForSaleSecurities | 5,895,436,000 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 | 93,900,000 cfr_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentPreTax | 129,300,000 cfr_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentPreTax |
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax | 61,000,000 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax | 84,100,000 us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax |
Reclassified To Held To Maturity [Member] |  |  |
Schedule Of Marketable Securities [Line Items] |  |  |
Securities available for sale, at estimated fair value |  |  | $ 2,300,000,000 us-gaap_AvailableForSaleSecurities / us-gaap_InformationByCategoryOfDebtSecurityAxis = cfr_ReclassifiedToHeldToMaturityMember

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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=28358780&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 11 -URI http://asc.fasb.org/extlink&oid=28358780&loc=d3e637-108580 Reference 3: 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=28358780&loc=d3e681-108580 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=28358780&loc=SL7669646-108580 + Details
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