Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ 1,984.8	$ 1,977.6	$ 1,984.8	$ 1,977.6	$ 1,959.8	$ 1,970.9	$ 1,998.9	$ 2,013.9
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax				0.0
Net unrealized gain (loss) on derivatives	2.4	0.0	1.7	0.2	(0.2)
Net unrealized (loss) gain on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	(4.0)		(4.0)		56.8
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax			16.6
Net unrealized gain (loss) on derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Net unrealized gain (loss) on derivatives			1.7
Net accumulated other comprehensive gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ 14.3	$ 61.9	$ 14.3	$ 61.9	$ 56.6	$ 24.1	$ 65.2	$ 11.0