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Shareholders' Equity (Details) - USD ($) |  | 3 Months Ended | 9 Months Ended
--- | --- | --- | ---
Aug. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017
Business Acquisition Equity Interests Issued Or Issuable [Line Items] |  |  |  |  |
Number of shares authorized to be repurchased |  | 6,000,000 |  | 6,000,000 |
Purchase of treasury stock |  | $ 16,600,000 | $ 0 | $ 62,500,000 | $ 13,000,000
Treasury Stock, Shares, Acquired |  | 300,000 |  | 1,100,000 | 300,000
Treasury Stock Acquired, Average Cost Per Share |  | $ 53.93 |  | $ 55.66 | $ 43.83
Business Bancshares, Inc. [Member] |  |  |  |  |
Business Acquisition Equity Interests Issued Or Issuable [Line Items] |  |  |  |  |
Shares issued |  |  |  | 4,800,000 |
Common stock reissued |  |  |  | 1,100,000 |
Stock issued for acquisition | 2,000,000 |  |  |  |

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