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, 2016	Mar. 31, 2015
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain/(loss) on sale of securities	$ 1.3	$ 3.8
Tax (expense) benefit	(0.8)	(8.5)
Total reclassifications for the period	0.2	1.7
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	0.8	2.7
Benefit Plan Items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(0.6)	(1.0)
Reclassification Out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	0.2	1.7
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	1.3	4.2
Total before tax	1.3	4.2
Tax (expense) benefit	(0.5)	(1.5)
Net of tax	0.8	2.7
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	1.4	1.4
Accumulated other comprehensive income components/Net actuarial loss	(2.4)	(2.9)
Total before tax	(1.0)	(1.5)
Tax (expense) benefit	0.4	0.5
Net of tax	$ (0.6)	$ (1.0)