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Income Taxes (Income Tax Expense Differences from the Amount Computed by Applying the U.S. Federal Income Tax Rate) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
U.S. Federal income tax rate (as a percent) | 21.00% | 21.00% | 21.00%
Reasons for the difference of income tax expense |  |  |
Computed expected tax expense | $ 45,218 | $ 55,086 | $ 57,831
Change in taxes resulting from: |  |  |
Tax-exempt interest income | (2,709) | (2,550) | (3,101)
State tax, net of federal income taxes and tax credit and refunds | 2,622 | 2,587 | 2,673
Other investment income | (2,205) | (1,480) | (1,561)
Deferred tax adjustment due to federal tax rate change |  |  | (1,618)
Net investment expense, low income housing investment | 1,990 | 623 | 2,518
Other | (477) | 584 | (90)
Total income taxes | $ 44,439 | $ 54,850 | $ 56,652

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