Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Income by Component, Net of Tax (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ (37.9)	$ (96.4)
Other comprehensive income before reclassifications	24.5	27.3
Amounts reclassified from AOCI	(0.3)	0.5
Other comprehensive income	24.2	27.8
Ending balance	(13.7)	(68.6)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Amounts reclassified from AOCI	(0.3)	0.5
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	40.4	(20.2)
Other comprehensive income before reclassifications	24.5	27.3
Amounts reclassified from AOCI	(1.9)	(0.1)
Other comprehensive income	22.6	27.2
Ending balance	63.0	7.0
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(78.3)	(76.2)
Other comprehensive income before reclassifications	0.0	0.0
Amounts reclassified from AOCI	1.6	0.6
Other comprehensive income	1.6	0.6
Ending balance	$ (76.7)	$ (75.6)