Deposits - Additional Information About Corporation's Deposits (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Disclosure Text Block [Abstract]
Deposits from the Certificate of Deposit Account Registry Service (CDARS) deposits	$ 369	$ 361
Deposits from foreign sources (primarily Mexico)	845,950	805,828
Deposits not covered by deposit insurance	$ 17,265,358	$ 13,115,796

This element represents Certificate Of Deposit Account Registry Service (CDARS) deposits

No definition available.

Name:	cfr_CertificateOfDepositAccountRegistryServiceCdarsDeposits
Namespace Prefix:	cfr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The aggregate of all foreign interest-bearing and noninterest-bearing deposit liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-05(a))
-URI http://asc.fasb.org/extlink&oid=120399901&loc=d3e537907-122884

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.12(a))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_DepositsForeign
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	us-gaap_DisclosureTextBlockAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of time deposit liabilities, including certificates of deposit, in denominations that meet or exceed the Federal Deposit Insurance Corporation (FDIC) insurance limit.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=116652737&loc=d3e64164-112818

+ Details

Name:	us-gaap_TimeDepositsAtOrAboveFDICInsuranceLimit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant