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Selected Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Interest income | $ 67,855 | $ 66,723 | $ 64,914 | $ 61,150 | $ 59,096 | $ 57,808 | $ 55,850 | $ 55,019 | $ 260,642 | $ 227,774 | $ 203,548
Interest expense | 10,027 | 9,267 | 8,344 | 7,272 | 6,638 | 6,353 | 6,142 | 5,382 | 34,909 | 24,515 | 15,997
Provision for loan losses | 982 | 2,850 | 4,869 | 5,183 | 5,586 | 2,516 | 4,848 | 5,014 | 13,883 | 17,965 | 10,388
Net Interest Income After Provision for Loan Losses | 56,846 | 54,606 | 51,701 | 48,695 | 46,872 | 48,939 | 44,860 | 44,623 | 211,850 | 185,294 | 177,163
Security (losses) gains, net | (986) | 0 | 3,617 | 370 | 0 | 0 | 0 | 0 |  |  |
Noninterest income | 13,636 | 13,551 | 12,648 | 12,626 | 12,922 | 13,448 | 12,448 | 15,817 | 55,462 | 54,635 | 51,033
Noninterest expense | 37,947 | 36,553 | 36,597 | 36,808 | 35,625 | 34,439 | 34,753 | 38,416 | 147,907 | 143,232 | 136,717
Income Before Taxes | 31,549 | 31,604 | 31,369 | 24,883 | 24,169 | 27,948 | 22,555 | 22,024 |  |  |
Provision for income taxes | 22,255 | 8,883 | 8,604 | 6,695 | 6,510 | 7,367 | 5,496 | 5,931 | 46,437 | 25,305 | 24,398
Net Income Allocated to Common Shareholders | $ 9,294 | $ 22,721 | $ 22,765 | $ 18,188 | $ 17,659 | $ 20,581 | $ 17,059 | $ 16,093 | $ 72,726 | $ 71,167 | $ 66,801
Per Share Data |  |  |  |  |  |  |  |  |  |  |
Common earnings per share-diluted (in USD per share) | $ 0.27 | $ 0.65 | $ 0.65 | $ 0.52 | $ 0.51 | $ 0.59 | $ 0.49 | $ 0.46 | $ 2.09 | $ 2.05 | $ 1.98
Dividends declared per common share (in USD per share) | 0.22 | 0.20 | 0.20 | 0.20 | 0.20 | 0.19 | 0.19 | 0.19 | $ 0.82 | $ 0.77 | $ 0.73
Common book value (in USD per share) | $ 25.28 | $ 25.37 | $ 24.90 | $ 24.45 | $ 24.12 | $ 24.02 | $ 23.63 | $ 23.23 |  |  |

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