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Parent Company Condensed Financial Information - Statements of Net Income (Details) - USD ($) $ 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
Condensed Income Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total 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 on long-term debt |  |  |  |  |  |  |  |  | 463 | 670 | 790
Total Expense | 37,947 | 36,553 | 36,597 | 36,808 | 35,625 | 34,439 | 34,753 | 38,416 | 147,907 | 143,232 | 136,717
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 |  |  |  |  |  |  |  |  | 72,968 | 71,392 | 67,081
Parent Company |  |  |  |  |  |  |  |  |  |  |
Condensed Income Statements, Captions [Line Items] |  |  |  |  |  |  |  |  |  |  |
Dividends from subsidiaries |  |  |  |  |  |  |  |  | 36,169 | 34,134 | 75,413
Investment income |  |  |  |  |  |  |  |  | 22 | 17 | 19
Total Income |  |  |  |  |  |  |  |  | 36,191 | 34,151 | 75,432
Interest expense on long-term debt |  |  |  |  |  |  |  |  | 955 | 854 | 773
Other expenses |  |  |  |  |  |  |  |  | 3,801 | 4,012 | 3,687
Total Expense |  |  |  |  |  |  |  |  | 4,756 | 4,866 | 4,460
Income before income tax and undistributed net income of subsidiaries |  |  |  |  |  |  |  |  | 31,435 | 29,285 | 70,972
Provision for income taxes |  |  |  |  |  |  |  |  | (1,596) | (1,697) | (1,549)
Income before income tax and undistributed net income of subsidiaries |  |  |  |  |  |  |  |  | 33,031 | 30,982 | 72,521
Bank subsidiary |  |  |  |  |  |  |  |  | 40,877 | 40,051 | (5,064)
Nonbank subsidiaries |  |  |  |  |  |  |  |  | (940) | 359 | (376)
Net Income |  |  |  |  |  |  |  |  | $ 72,968 | $ 71,392 | $ 67,081

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