Accumulated Other Comprehensive Income - Summary of Changes in Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	$ 19.9	$ 37.5	$ 92.3	$ (25.0)
Other comprehensive income before reclassifications	(44.6)	(36.3)	27.7	(48.9)
Amounts reclassified from accumulated other comprehensive income	0.0	(0.1)	(0.1)	(0.2)
Net other comprehensive income (loss)	(44.6)	(36.2)	27.8	(48.7)
Ending balance	64.5	73.7	64.5	73.7
Unrealized holding gains (losses) on AFS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	20.0	27.6	92.1	(21.4)
Other comprehensive income before reclassifications	(44.8)	(39.3)	27.2	(45.6)
Amounts reclassified from accumulated other comprehensive income	0.0	(0.1)	(0.1)	(0.2)
Net other comprehensive income (loss)	(44.8)	(39.2)	27.3	(45.4)
Ending balance	64.8	66.8	64.8	66.8
Accumulated Gain (Loss), Financial Liability, Fair Value Option, Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	0.2	10.2	0.5	3.6
Other comprehensive income before reclassifications	(0.2)	3.0	0.5	3.6
Amounts reclassified from accumulated other comprehensive income	0.0	0.0	0.0	0.0
Net other comprehensive income (loss)	(0.2)	3.0	0.5	3.6
Ending balance	0.0	7.2	0.0	7.2
Supplemental Employee Retirement Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance	0.3	(0.3)	0.3	(0.0)
Other comprehensive income before reclassifications	0.0	(0.0)	0.0	(0.3)
Amounts reclassified from accumulated other comprehensive income	0.0	0.0	0.0	0.0
Net other comprehensive income (loss)	0.0	0.0	0.0	(0.3)
Ending balance	$ 0.3	$ (0.3)	$ 0.3	$ (0.3)