Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ 1,970.9	$ 1,998.9	$ 1,933.4	$ 1,959.8	$ 1,956.1	$ 2,013.9
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax		0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent				(0.2)
Net unrealized gains on investment securities available-for-sale
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	6.3			56.8
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax	18.5
Net unrealized loss on derivatives
Accumulated Other Comprehensive Income (Loss) [Line Items]
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.7)
Net accumulated other comprehensive gains
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total stockholders’ equity	$ 24.1	$ 65.2	$ 6.7	$ 56.6	$ 39.8	$ 11.0