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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	$ 17.9	$ (41.7)	$ (37.9)	$ (96.4)
Other comprehensive income before reclassifications	3.5	17.8	58.8	72.5
Amounts reclassified from AOCI	1.9	(0.3)	2.4	(0.3)
Other comprehensive income	5.4	17.5	61.2	72.2
Ending balance	23.3	(24.2)	23.3	(24.2)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Amounts reclassified from AOCI	1.9	(0.3)	2.4	(0.3)
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	93.0	33.2	40.4	(20.2)
Other comprehensive income before reclassifications	3.5	17.8	58.8	72.5
Amounts reclassified from AOCI	0.2	(0.9)	(2.5)	(2.2)
Other comprehensive income	3.7	16.9	56.3	70.3
Ending balance	96.7	50.1	96.7	50.1
Benefit Plan Items [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Beginning balance	(75.1)	(74.9)	(78.3)	(76.2)
Other comprehensive income before reclassifications	0.0	0.0	0.0	0.0
Amounts reclassified from AOCI	1.7	0.6	4.9	1.9
Other comprehensive income	1.7	0.6	4.9	1.9
Ending balance	$ (73.4)	$ (74.3)	$ (73.4)	$ (74.3)