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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	$ 3,100,000,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral	$ 3,000,000,000us-gaap_AvailableForSaleSecuritiesPledgedAsCollateral
Available for Sale, Estimated Fair Value	8,672,497,000us-gaap_AvailableForSaleSecurities	8,461,254,000us-gaap_AvailableForSaleSecurities
Available for sale securities reclassified to held to maturity, unrealized gain			165,700,000cfr_AvailableForSaleSecuritiesTransfersToHeldToMaturityUnrealizedGains
Available for sale securities reclassified to held to maturity, unrealized gain net of tax			107,700,000cfr_AvailableForSaleSecuritiesTransferredToHeldToMaturitySecuritiesUnrealizedGainsNetOfTax
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax	86,000,000cfr_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentPreTax
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	55,900,000us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Available for Sale, Estimated Fair Value			$ 2,300,000,000us-gaap_AvailableForSaleSecurities / us-gaap_InformationByCategoryOfDebtSecurityAxis = cfr_ReclassifiedToHeldToMaturityMember

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Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Pre Tax

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Available For Sale Securities Transferred To Held To Maturity Securities Unrealized Gains Net Of Tax

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Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas

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Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas Guaranteed By Texas Permanent School Fund

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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

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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