| Component: (Network and Table) | |
|---|---|
| Network | 1088 - Disclosure - Pension and Profit Sharing Plans - Pension Plan and Targeted Allocation Percentage (Detail) (http://www.ffin.com/taxonomy/role/DisclosurePensionAndProfitSharingPlansPensionPlanAndTargetedAllocationPercentage) |
| Table | Schedule of Defined Benefit Plans Disclosures [Table] |
| Reporting Entity [Axis] | 0000036029 (http://www.sec.gov/CIK) |
| Defined Benefit Plan Disclosure [Line Items] | Period [Axis] | |||||
|---|---|---|---|---|---|---|
2016-01-01 - 2016-12-31 | 2015-12-31 | |||||
| Defined Benefit Plan, Asset Categories [Axis] | Defined Benefit Plan, Asset Categories [Axis] | |||||
| Equity Securities [Member] | Debt Securities [Member] | Cash and Equivalents [Member] | Equity Securities [Member] | Debt Securities [Member] | Cash and Equivalents [Member] | |
Allocation | ||||||
Targeted Allocation | ||||||