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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Sep. 30, 2015 | Dec. 31, 2014
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Cullen/Frost [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount | $ 1,979,694 |
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 11.57% |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 769,754 |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio | 4.50% |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,197,002 |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio | 7.00% |
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount | $ 1,111,867 |
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio | 6.50% |
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,157,430 | $ 2,185,558
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.61% | 13.67%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 1,026,339 |
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio | 6.00% |
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,453,503 |
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 8.50% |
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,368,452 | $ 959,098
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 8.00% | 6.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,387,553 | $ 2,325,100
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.96% | 14.55%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 1,368,452 |
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio | 8.00% |
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,795,503 |
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 10.50% |
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,710,565 | $ 1,598,496
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 2,157,430 | $ 2,185,558
Leverage Ratio, Actual, Ratio | 7.91% | 8.16%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 1,091,452 |
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio | 4.00% |
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,091,227 |
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 4.00% |
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 1,364,315 | $ 1,340,043
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount |  | $ 639,398
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio |  | 4.00%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount |  | $ 1,278,797
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio |  | 8.00%
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Capital Amount |  | $ 1,072,035
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Ratio |  | 4.00%
Frost Bank [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount | $ 2,111,113 |
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 12.38% |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 767,137 |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio | 4.50% |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,192,930 |
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio | 7.00% |
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount | $ 1,108,086 |
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio | 6.50% |
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,111,113 | $ 1,978,697
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.38% | 12.41%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 1,022,849 |
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio | 6.00% |
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,448,558 |
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 8.50% |
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,363,798 | $ 956,894
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 8.00% | 6.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 2,221,486 | $ 2,070,293
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.03% | 12.98%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 1,363,798 |
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio | 8.00% |
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,789,396 |
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 10.50% |
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,704,748 | $ 1,594,823
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 2,111,113 | $ 1,978,697
Leverage Ratio, Actual, Ratio | 7.75% | 7.40%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount | $ 1,090,250 |
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio | 4.00% |
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,090,025 |
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 4.00% |
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 1,362,812 | $ 1,337,636
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount |  | $ 637,929
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio |  | 4.00%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount |  | $ 1,275,858
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio |  | 8.00%
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Capital Amount |  | $ 1,070,109
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Ratio |  | 4.00%

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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=35709795&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=35709795&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 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=35709795&loc=SL5958568-112826 + Details
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- Definition Tier 1 Leverage Capital divided by average assets as defined in the regulations. + 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=35709795&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=35709795&loc=d3e65071-112826 + Details
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- Definition Tier 1 Risk Based Capital as defined in the regulations. + 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=35709795&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=35709795&loc=d3e65071-112826 + Details
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- Definition The minimum amount of Tier 1 Risk Based Capital required for capital adequacy purposes 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 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826 Reference 2: 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=35709795&loc=SL5958568-112826 + Details
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- Definition The minimum Tier One Capital Ratio (Tier one capital divided by risk-weighted assets) required for capital adequacy purposes 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=35709795&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 1B -URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958570-112826 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=35709795&loc=d3e65071-112826 + 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=35709795&loc=SL5958568-112826 + Details
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