Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	$ (135.2)	$ 44.9
Tax (expense) benefit	29.4	(12.2)
Total reclassifications for the period	0.3	(0.5)
Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	1.9	0.1
Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(1.6)	(0.6)
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	0.3	(0.5)
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Unrealized Gains and Losses on Available-for-Sale Securities [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	2.4	0.1
Total before tax	2.4	0.1
Tax (expense) benefit	(0.5)	0.0
Net of tax	1.9	0.1
Reclassification Out of Accumulated Other Comprehensive Income [Member] | Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive income components/Negative prior-service costs	1.6	1.6
Accumulated other comprehensive income components/Net actuarial loss	(3.7)	(2.4)
Total before tax	(2.1)	(0.8)
Tax (expense) benefit	0.5	0.2
Net of tax	$ (1.6)	$ (0.6)