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Other Non-Interest Income and Expense (Narrative) (Detail) | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Other Income and Expenses [Abstract] |  |  |
Minimum percentage of the aggregate total net interest income and total non interest income required for amounts to be stated separately | 1.00% | 1.00% | 1.00%

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- Definition Minimum Percentage Of Aggregate Net Interest Income And Non Interest Income Required For Amounts To Be Stated Separately + References No definition available. + Details
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Namespace Prefix: | cfr_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_OtherIncomeAndExpensesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration