Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Condensed balance sheets:
Cash and cash equivalents	$ 2,276.8	$ 1,076.8
Investment in subsidiaries, at equity:
Other assets	215.3	187.6
Total assets	17,648.7	14,644.2
Long-term debt	98.6	0.0
Subordinated debentures held by subsidiary trusts	87.0	86.9
Total liabilities	15,688.9	12,630.3
Stockholders’ equity	1,959.8	2,013.9
Total liabilities and stockholders’ equity	17,648.7	14,644.2
Parent Company
Condensed balance sheets:
Cash and cash equivalents	123.2	147.1
Investment in subsidiaries, at equity:
Equity method investments	1,986.6	1,899.3
Advances from subsidiaries, net	41.0	48.9
Other assets	61.3	61.7
Total assets	2,212.1	2,157.0
Other liabilities	66.7	56.2
Subordinated debentures held by subsidiary trusts	87.0	86.9
Total liabilities	252.3	143.1
Stockholders’ equity	1,959.8	2,013.9
Total liabilities and stockholders’ equity	$ 2,212.1	$ 2,157.0