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