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Income Taxes - Effective Income Tax Reconciliation (Details) - USD ($) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 22, 2017 | Dec. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Percent |  |  |  |
Federal statutory income tax rate |  |  | 21.00% | 35.00%
California franchise tax, net of federal tax benefit |  |  | 8.00% | 6.90%
Write down of federal deferred tax assets, net |  |  | 0.00% | 10.50%
Tax exempt interest on municipal securities and loans |  |  | (2.40%) | (6.10%)
Tax exempt earnings on bank owned life insurance |  |  | (0.40%) | (1.00%)
Non-deductible acquisition related expenses |  |  | 0.00% | 0.80%
Low income housing and qualified zone academy bond tax credits |  |  | (0.50%) | (0.40%)
Stock-based compensation excess tax benefit |  |  | (0.60%) | (0.30%)
Other |  |  | (0.20%) | (0.80%)
Effective Tax Rate |  |  | 24.90% | 44.60%
Write-down to net deferred tax assets recorded as income tax expense | $ 3.0 | $ 3.0 |  |

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