Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2016
Condensed balance sheets:
Cash and cash equivalents	$ 1,076.8	$ 822.0
Investment in subsidiaries, at equity:
Other assets	187.6	159.1
Total assets	14,644.2	13,300.2
Subordinated debentures held by subsidiary trusts	86.9	86.9
Total liabilities	12,630.3	11,606.3
Stockholders’ equity	2,013.9	1,693.9	$ 982.6
Total liabilities and stockholders’ equity	14,644.2	13,300.2
Parent Company
Condensed balance sheets:
Cash and cash equivalents	147.1	72.9
Investment in subsidiaries, at equity:
Equity method investments	1,899.3	1,648.9
Advances from subsidiaries, net	48.9	46.0
Other assets	61.7	59.0
Total assets	2,157.0	1,826.8
Other liabilities	56.2	46.0
Subordinated debentures held by subsidiary trusts	86.9	86.9
Total liabilities	143.1	132.9
Stockholders’ equity	2,013.9	1,693.9
Total liabilities and stockholders’ equity	$ 2,157.0	$ 1,826.8