Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 1,774.1	$ 1,477.0	$ 1,439.5	$ 1,234.5	$ 1,234.5
Investment securities, pre tax	122.7	146.8	554.4	470.4
Net non-credit related OTTI losses, adjusted for valuation changes, pre tax	0.0	0.0	0.0	(0.1)
Forecasted transactions, pre tax	0.0	0.0	(8.0)	0.0
Foreign currency translation adjustment, pre tax	0.9	0.4	1.2	1.0
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	(1.1)	1.8	(4.5)	(3.7)
Total other comprehensive income (loss) before reclassifications, pre tax	122.5	149.0	543.1	467.6
Total reclassification adjustment for amounts realized in net income, pre tax	6.2	13.3	92.2	89.4
Total other comprehensive income (loss), pre tax	116.3	135.7	450.9	378.2
AOCI before Tax, Attributable to Parent, ending balance	1,890.4	1,612.7	1,890.4	1,612.7	1,439.5
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(623.4)	(519.2)	(506.1)	(434.1)	(434.1)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	(43.0)	(51.4)	(194.3)	(165.2)
Net non-credit related OTTI losses, adjusted for valuation changes, Total tax (provision) benefit	0.0	0.0	0.0	(0.1)
Forecasted transactions, Total tax (provision) benefit	0.0	0.0	2.8	0.0
Foreign currency translation adjustment, Total tax (provision) benefit	(0.3)	(0.1)	(0.4)	(0.3)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	0.4	(0.6)	1.6	1.4
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax	(42.9)	(52.1)	(190.3)	(164.0)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(2.1)	(4.7)	(32.2)	(31.5)
Total other comprehensive income (loss), Total tax (provision) benefit	(40.8)	(47.4)	(158.1)	(132.5)
AOCI Tax, Attributable to Parent, ending balance	(664.2)	(566.6)	(664.2)	(566.6)	(506.1)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,150.7	957.8	933.4	800.4	800.4
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	79.7	95.4	360.1	305.2
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	(5.2)	0.0
Foreign currency translation adjustment, after tax	0.6	0.3	0.8	0.7
Other comprehensive (income) loss attributable to NCI	(0.7)	1.2	(2.9)	(2.3)	3.2
Total other comprehensive income (loss) before reclassifications, after tax	79.6	96.9	352.8	303.6
Total reclassification adjustment for amounts realized in net income, after tax	4.1	8.6	60.0	57.9
Other comprehensive income (loss)	75.5	88.3	292.8	245.7
After tax total accumulated other comprehensive income, ending balance	1,226.2	1,046.1	1,226.2	1,046.1	933.4
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	9.3	26.8	12.9	27.0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	3.3	9.4	4.6	9.5
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	6.0	17.4	8.3	17.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	15.7	39.6	104.5	114.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(5.5)	(13.9)	(36.6)	(40.4)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	10.2	25.7	67.9	74.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.2)	0.5	0.6	1.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.1	(0.2)	(0.2)	(0.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.1)	0.3	0.4	0.9
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,164.6	970.1	939.6	809.0	809.0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	79.7	95.4	360.1	305.2
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment, after tax	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) before reclassifications, after tax	79.7	95.4	360.1	305.2
Total reclassification adjustment for amounts realized in net income, after tax	4.2	8.3	59.6	57.0
Other comprehensive income (loss)	75.5	87.1	300.5	248.2
After tax total accumulated other comprehensive income, ending balance	1,240.1	1,057.2	1,240.1	1,057.2	939.6
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	6.0	17.4	8.3	17.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	10.2	25.7	67.9	74.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.0	0.0	0.0
Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(15.1)	(8.8)	(9.4)	(8.2)	(8.2)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0.0	0.0	0.0	0.0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	(5.2)	0.0
Foreign currency translation adjustment, after tax	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) before reclassifications, after tax	0.0	0.0	(5.2)	0.0
Total reclassification adjustment for amounts realized in net income, after tax	(0.1)	0.3	0.4	0.9
Other comprehensive income (loss)	0.1	(0.3)	(5.6)	(0.9)
After tax total accumulated other comprehensive income, ending balance	(15.0)	(9.1)	(15.0)	(9.1)	(9.4)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.1)	0.3	0.4	0.9
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(0.9)	(1.1)	(1.1)	(1.5)	(1.5)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0.0	0.0	0.0	0.0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment, after tax	0.6	0.3	0.8	0.7
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Total other comprehensive income (loss) before reclassifications, after tax	0.6	0.3	0.8	0.7
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	0.6	0.3	0.8	0.7
After tax total accumulated other comprehensive income, ending balance	(0.3)	(0.8)	(0.3)	(0.8)	(1.1)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.0	0.0	0.0
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	2.1	(2.4)	4.3	1.1	1.1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0.0	0.0	0.0	0.0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment, after tax	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	(0.7)	1.2	(2.9)	(2.3)
Total other comprehensive income (loss) before reclassifications, after tax	(0.7)	1.2	(2.9)	(2.3)
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Other comprehensive income (loss)	(0.7)	1.2	(2.9)	(2.3)
After tax total accumulated other comprehensive income, ending balance	1.4	(1.2)	1.4	(1.2)	$ 4.3
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ 0.0	$ 0.0	$ 0.0	$ 0.0