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Income Taxes - Reconciliation of the Federal Statutory Tax Rate to the Total Tax Provision (Detail) |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2018 | Dec. 31, 2017 | Sep. 30, 2019 | Sep. 30, 2018
Income Tax Disclosure [Abstract] |  |  |  |
Federal income taxes computed at the statutory rate | 21.00% | 35.00% | 21.00% | 24.30%
State income taxes, net of federal benefit |  |  | 1.60% | 1.40%
Change in current tax rate |  |  |  | (2.90%)
Change in deferred tax rate |  |  |  | (2.30%)
Research & development tax refunds & credits |  |  | (1.90%) | (1.80%)
Dividend received deduction |  |  | (0.60%) | (0.90%)
Domestic production activities deduction |  |  |  | (1.20%)
Incentive stock options |  |  | 0.20% | (1.00%)
Other, net |  |  | 0.20% | 0.10%
Effective income tax rate |  |  | 20.50% | 15.70%

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