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Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Cullen/Frost [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common equity tier one to risk-weighted assets, actual, capital amount | $ 2,426,048 | $ 2,239,186
Common equity tier one to risk-weighted assets, actual, ratio | 12.42% | 12.52%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount | $ 1,123,430 | $ 916,360
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 5.75% | 5.125%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,367,583 | $ 1,251,425
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 7.00% | 7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount | $ 1,269,965 | $ 1,162,213
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio | 6.50% | 6.50%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,959,326 | $ 2,669,717
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.15% | 14.93%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount | $ 1,807,257 | $ 1,542,168
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 9.25% | 8.625%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,051,375 | $ 1,877,137
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,953,792 | $ 1,788,020
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,570,534 | $ 2,383,672
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.16% | 13.33%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount | $ 1,416,499 | $ 1,184,563
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 7.25% | 6.625%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,660,637 | $ 1,519,587
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,563,033 | $ 1,430,416
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 8.00% | 8.00%
Leverage Ratio, Actual, Capital Amount | $ 2,570,534 | $ 2,383,672
Leverage Ratio, Actual, Ratio | 8.46% | 8.14%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount | $ 1,215,227 | $ 1,171,682
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 4.00% | 4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,215,186 | $ 1,171,573
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 1,519,034 | $ 1,464,602
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%
Frost Bank [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common equity tier one to risk-weighted assets, actual, capital amount | $ 2,518,999 | $ 2,296,480
Common equity tier one to risk-weighted assets, actual, ratio | 12.92% | 12.88%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount | $ 1,120,663 | $ 913,460
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 5.75% | 5.125%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,364,214 | $ 1,247,463
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 7.00% | 7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount | $ 1,266,836 | $ 1,158,535
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio | 6.50% | 6.50%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,674,791 | $ 2,449,525
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.72% | 13.74%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount | $ 1,802,805 | $ 1,537,286
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 9.25% | 8.625%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,046,321 | $ 1,871,194
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,948,979 | $ 1,782,361
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,518,999 | $ 2,296,480
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.92% | 12.88%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount | $ 1,413,010 | $ 1,180,814
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 7.25% | 6.625%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,656,546 | $ 1,514,776
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,559,183 | $ 1,425,889
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 8.00% | 8.00%
Leverage Ratio, Actual, Capital Amount | $ 2,518,999 | $ 2,296,480
Leverage Ratio, Actual, Ratio | 8.30% | 7.85%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount | $ 1,214,295 | $ 1,170,249
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule, Ratio | 4.00% | 4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,214,254 | $ 1,170,141
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 1,517,869 | $ 1,462,812
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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- Definition The amount of total capital as defined in the regulations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47304-110998 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 948 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998 + Details
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- Definition The amount of Total Capital required to be categorized as well as capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition Amount of Common Equity Tier One Risk-based Capital, as defined by regulatory framework. + References No definition available. + Details
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- Definition The amount of excess Tier 1 Risk Based Capital (which consists of the types of financial capital considered the most reliable and liquid, primarily Shareholders' equity. Examples of Tier 1 capital are common stock or unit, preferred stock or unit that is irredeemable and noncumulative, and retained earnings) divided by risk-weighted assets which are assets that are weighted for credit risk according to a formula used by the Reserve Bank. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition Tier 1 Leverage Capital as defined in the regulations. Also called Core Capital. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=84167132&loc=d3e65071-112826 + Details
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- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The amount of Tier 1 Risk Based Capital required to be categorized as well capitalized under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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- Definition The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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