Other Comprehensive Income and Accumulated Other Comprehensive Income - Schedule of Reclassifications Out of Accumulated Other Comprehensive Income by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	$ 24.3	$ 20.7	$ 23.2
Tax (expense) benefit	(16.1)	80.6	(0.5)
Total reclassifications for the period	(14.0)	6.4	(19.8)
Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(14.0)	6.4	(19.8)
Unrealized Gains and Losses on Available-for-Sale Securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	(16.1)	(13.5)	(23.3)
Unrealized Gains and Losses on Available-for-Sale Securities | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	24.7	20.7	23.2
Total before tax	24.7	20.7	23.2
Tax (expense) benefit	(8.6)	(7.2)	0.1
Total reclassifications for the period	16.1	13.5	23.3
Gains and Losses on Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	0.0	0.0	(0.1)
Gains and Losses on Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Realized gain on sale of securities	0.0	0.0	0.1
Total before tax	0.0	0.0	0.1
Tax (expense) benefit	0.0	0.0	0.0
Total reclassifications for the period	0.0	0.0	0.1
Benefit Plan Items
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period	2.1	19.9	3.6
Benefit Plan Items | Reclassification out of Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(6.1)	(1.4)	(3.6)
Tax (expense) benefit	4.0	(18.5)	0.0
Negative prior service costs	5.4	5.5	5.5
Net loss	(11.5)	(6.9)	(9.1)
Total reclassifications for the period	$ (2.1)	$ (19.9)	$ (3.6)