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Acquisitions - Schedule of Unaudited Pro Forma Revenue and Income (Details) - Cascade Bank - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Business Acquisition [Line Items] |  |
Interest income | $ 420.8 | $ 392.7
Non-interest income | 153.3 | 165.9
Total revenues | 574.1 | 558.6
Net income | $ 126.4 | $ 72.3
EPS - basic (in dollars per share) | $ 2.03 | $ 1.30
EPS - diluted (in dollars per share) | $ 2.01 | $ 1.29

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