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Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Cash flows from operating activities: |  |  |  |
Net income | $ 160.2 | $ 181.0 | $ 160.2 | $ 106.5
Adjustments to reconcile net income to cash provided by operating activities: |  |  |  |
Stock-based compensation expense |  | 8.0 | 5.6 | 3.9
Net cash provided by operating activities |  | 127.3 | 219.0 | 154.6
Cash flows from investing activities: |  |  |  |
Acquisition of intangible assets |  | 0.0 | 0.0 | (28.0)
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired |  | 298.4 | (28.1) | 91.8
Net cash used in investing activities |  | (16.8) | 181.1 | (130.6)
Cash flows from financing activities: |  |  |  |
Proceeds from issuance of common stock, net of stock issuance costs |  | 1.0 | 1.8 | 2.4
Purchase and retirement of common stock |  | (2.5) | (1.0) | (1.3)
Dividends paid to common stockholders |  | (79.2) | (64.1) | (48.6)
Net cash used in financing activities |  | 144.3 | 25.2 | (47.1)
Net change in cash and cash equivalents |  |  |  | (23.1)
Cash and cash equivalents at beginning of period | 758.9 |  | 758.9 | 782.0
Cash and cash equivalents at end of period |  |  |  | 758.9
Parent Company |  |  |  |
Cash flows from operating activities: |  |  |  |
Net income |  | 181.0 | 160.2 | 106.5
Adjustments to reconcile net income to cash provided by operating activities: |  |  |  |
Undistributed earnings of subsidiaries |  | (35.6) | (37.7) | 11.0
Stock-based compensation expense |  | 8.0 | 5.6 | 3.9
Other, net |  | 8.1 | 16.4 | 14.7
Net cash provided by operating activities |  | 161.5 | 144.5 | 136.1
Cash flows from investing activities: |  |  |  |
Capital distributions from nonbank subsidiaries |  | 0.0 | 0.0 | 18.0
Acquisition of intangible assets |  | 0.0 | 0.0 | (28.0)
Acquisition of banks and bank holding companies, net of cash and cash equivalents acquired |  | 0.0 | (14.7) | (128.3)
Investment in subsidiary |  | 0.0 | 0.0 | (18.0)
Net cash used in investing activities |  | 0.0 | (14.7) | (156.3)
Cash flows from financing activities: |  |  |  |
Net (decrease) increase in advances from subsidiaries |  | (6.6) | (9.9) | (28.4)
Repayment of long-term debt |  | 0.0 | (26.0) | 0.0
Proceeds from issuance of common stock, net of stock issuance costs |  | 1.0 | 1.8 | 2.4
Purchase and retirement of common stock |  | (2.5) | (1.0) | (1.3)
Dividends paid to common stockholders |  | (79.2) | (64.1) | (48.6)
Net cash used in financing activities |  | (87.3) | (99.2) | (75.9)
Net change in cash and cash equivalents |  | 74.2 | 30.6 | (96.1)
Cash and cash equivalents at beginning of period | $ 42.3 | 72.9 | 42.3 | 138.4
Cash and cash equivalents at end of period |  | 147.1 | 72.9 | 42.3
Community 1st Bank [Member] |  |  |  |
Cash flows from financing activities: |  |  |  |
Stock Issued |  | $ 176.1 |  |
Inland Northwest Bank [Member] |  |  |  |
Cash flows from financing activities: |  |  |  |
Stock Issued |  |  | $ 173.3 |
BOTC Acquisition |  |  |  |
Cash flows from financing activities: |  |  |  |
Stock Issued |  |  |  | $ 386.0

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