Stock-Based Compensation Plans - Narrative (Details) $ in Millions		3 Months Ended		9 Months Ended
	Jul. 01, 2022 shares	Sep. 30, 2025 USD ($) plan shares	Sep. 30, 2024 USD ($)	Sep. 30, 2025 USD ($) plan shares	Sep. 30, 2024 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense for share-based payments		$ 10.6	$ 9.6	$ 27.0	$ 31.0
Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total grants in period (in shares) | shares				12,518,406
Shares issued under the omnibus plan (in shares) | shares				35,103,596
Number of shares authorized (in shares) | shares		45,000,000		45,000,000
Employee Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost		$ 4.2		$ 4.2
Employee Stock Option Plan | Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period				1 year
Option expiration period				5 years
Employee Stock Option Plan | Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period				5 years
Option expiration period				10 years
Restricted Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost		54.6		$ 54.6
Restricted period, maximum				10 years
Amortization expense		$ 6.1	5.9	$ 16.9	19.5
Gentex Corporation Employee Stock Purchase Plan | Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under the omnibus plan (in shares) | shares				730,907
Number of shares authorized (in shares) | shares	2,000,000
ESPP discount rate	85.00%
ESPP discount rate recognized as compensation expense	15.00%
Equity Incentive Plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of plans | plan		2		2
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense for share-based payments		$ 2.7	1.6	$ 4.7	5.4
Unrecognized compensation cost		11.3		$ 11.3
Performance Shares | Omnibus Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under the omnibus plan (in shares) | shares				5,132,575
Performance Share Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense for share-based payments		$ 0.5	$ 0.5	$ 1.4	$ 1.3
Option vesting period				4 years