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Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
--- | --- | --- | --- | ---
Condensed balance sheets: |  |  |  |
Cash and cash equivalents | $ 822.0 | $ 758.9 |  |
Investment in subsidiaries, at equity: |  |  |  |
Other assets | 159.1 | 145.8 |  |
Total assets | 13,300.2 | 12,213.3 |  |
Subordinated debentures held by subsidiary trusts | 86.9 | 82.5 |  |
Total liabilities | 11,606.3 | 10,785.7 |  |
Stockholders’ equity | 1,693.9 | 1,427.6 | $ 982.6 | $ 950.5
Total liabilities and stockholders’ equity | 13,300.2 | 12,213.3 |  |
Parent Company |  |  |  |
Condensed balance sheets: |  |  |  |
Cash and cash equivalents | 72.9 | 42.3 |  |
Investment in subsidiaries, at equity: |  |  |  |
Equity method investments | 1,648.9 | 1,432.3 |  |
Advances from subsidiaries, net | 46.0 | 25.7 |  |
Other assets | 59.0 | 61.7 |  |
Total assets | 1,826.8 | 1,562.0 |  |
Other liabilities | 46.0 | 51.9 |  |
Subordinated debentures held by subsidiary trusts | 86.9 | 82.5 |  |
Total liabilities | 132.9 | 134.4 |  |
Stockholders’ equity | 1,693.9 | 1,427.6 |  |
Total liabilities and stockholders’ equity | $ 1,826.8 | $ 1,562.0 |  |

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