Recent Authoritative Accounting Guidance (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total stockholders’ equity	$ 1,984.8	$ 1,970.9	$ 1,959.8	$ 1,977.6	$ 1,998.9	$ 2,013.9
Off-Balance Sheet, Credit Loss, Liability	3.2	3.4	3.7	3.5	2.3	0.0
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Total stockholders’ equity	$ 1,026.9	$ 1,005.2	$ 962.1	$ 938.9	$ 912.5	$ 953.6