Condensed Financial Information (Parent Company Only) - Condensed Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Condensed balance sheets:
Cash and cash equivalents	$ 2,344.8	$ 2,276.8
Investment in subsidiaries, at equity:
Other assets	237.9	215.3
Total assets	19,671.9	17,648.7
Long-term debt	98.7	98.6
Subordinated debentures held by subsidiary trusts	87.0	87.0
Total liabilities	17,685.3	15,688.9
Stockholders’ equity	1,986.6	1,959.8	$ 2,013.9	$ 1,693.9
Total liabilities and stockholders’ equity	19,671.9	17,648.7
Parent Company
Condensed balance sheets:
Cash and cash equivalents	181.1	123.2
Investment in subsidiaries, at equity:
Equity method investments	1,948.9	1,986.6
Advances from subsidiaries, net	5.9	41.0
Other assets	63.2	61.3
Total assets	2,199.1	2,212.1
Other liabilities	26.8	66.7
Subordinated debentures held by subsidiary trusts	87.0	87.0
Total liabilities	212.5	252.3
Stockholders’ equity	1,986.6	1,959.8
Total liabilities and stockholders’ equity	$ 2,199.1	$ 2,212.1