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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ (280.8)	$ 1,774.1	$ 1,977.8	$ 1,439.5	$ 1,439.5
Pretax Cumulative Effect of New Accounting Principal in Period of Adoption			(2,006.0)
Pretax Reclassification of Stranded Amounts in AOCI to Retained Earnings	(4.3)		(4.3)
AOCI before Tax, Attributable to Parent, after Cumulative Effect Adjustment	(285.1)		(32.5)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax	(58.7)	122.7	(352.5)	554.4
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0.0	0.0	0.0	(8.0)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax	0.0	0.9	0.0	1.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	0.6	(1.1)	6.4	(4.5)
OCI, before Reclassifications, before Tax, Attributable to Parent	(58.1)	122.5	(346.1)	543.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	0.0	(9.3)	0.0	(12.9)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	(9.3)	15.7	(44.2)	104.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.2)	(0.2)	(0.7)	0.6
Total reclassification adjustment for amounts realized in net income, pre tax	(9.5)	6.2	(44.9)	92.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(48.6)	116.3	(301.2)	450.9
AOCI before Tax, Attributable to Parent, ending balance	(333.7)	1,890.4	(333.7)	1,890.4	1,977.8
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	58.9	(623.4)	(695.6)	(506.1)	(506.1)
Tax, Cumulative Effect of New Accounting Principal in Period of Adoption			705.8
Tax, Reclassification of Stranded Amounts in AOCI to Retained Earnings	0.9		(3.4)
AOCI Tax, Attributable to Parent, After Cumulative Effect Adjustment	59.8		6.8
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	12.3	(43.0)	73.9	(194.3)
Forecasted transactions, Total tax (provision) benefit	0.0	0.0	0.0	2.8
Other Comprehensive Income (Loss), Foreign Current Translation Adjustment, Tax	0.0	(0.3)	0.0	(0.4)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	(0.2)	0.4	(1.4)	1.6
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax	12.1	(42.9)	72.5	(190.3)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	0.0	3.3	0.0	4.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	1.9	(5.5)	9.2	(36.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.0	0.1	0.1	(0.2)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	1.9	(2.1)	9.3	(32.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	10.2	(40.8)	63.2	(158.1)
AOCI Tax, Attributable to Parent, ending balance	70.0	(664.2)	70.0	(664.2)	(695.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(221.9)	1,150.7	1,282.2	933.4	933.4
Cumulative Effect of New Accounting Principle in Period of Adoption			(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings	(3.4)		(7.7)
AOCI, Net of Tax, After Cumulative Effect Adjustment	(225.3)		(25.7)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(46.4)	79.7	(278.6)	360.1
Forecasted transactions, after tax	0.0	0.0	0.0	(5.2)
Foreign currency translation adjustment	0.0	0.6	0.0	0.8
Other comprehensive (income) loss attributable to NCI	0.4	(0.7)	5.0	(2.9)	(2.3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(46.0)	79.6	(273.6)	352.8
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0.0	(6.0)	0.0	(8.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(7.4)	10.2	(35.0)	67.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	(0.1)	(0.6)	0.4
Total reclassification adjustment for amounts realized in net income, after tax	(7.6)	4.1	(35.6)	60.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(38.4)	75.5	(238.0)	292.8
After tax total accumulated other comprehensive income, ending balance	(263.7)	1,226.2	(263.7)	1,226.2	1,282.2
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	(210.9)	1,164.6	1,295.0	939.6	939.6
Cumulative Effect of New Accounting Principle in Period of Adoption			(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings	0.0		(1.1)
AOCI, Net of Tax, After Cumulative Effect Adjustment	(210.9)		(6.3)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(46.4)	79.7	(278.6)	360.1
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(46.4)	79.7	(278.6)	360.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0.0	(6.0)	0.0	(8.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(7.4)	10.2	(35.0)	67.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0	0.0	0.0	0.0
Total reclassification adjustment for amounts realized in net income, after tax	(7.4)	4.2	(35.0)	59.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(39.0)	75.5	(243.6)	300.5
After tax total accumulated other comprehensive income, ending balance	(249.9)	1,240.1	(249.9)	1,240.1	1,295.0
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(17.6)	(15.1)	(14.8)	(9.4)	(9.4)
Cumulative Effect of New Accounting Principle in Period of Adoption			0.0
Reclassification of Stranded Amounts in AOCI to Retained Earnings	0.0		(3.2)
AOCI, Net of Tax, After Cumulative Effect Adjustment	(17.6)		(18.0)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	(5.2)
Foreign currency translation adjustment	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0	0.0	0.0	(5.2)
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	(0.2)	(0.1)	(0.6)	0.4
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	(0.1)	(0.6)	0.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.2	0.1	0.6	(5.6)
After tax total accumulated other comprehensive income, ending balance	(17.4)	(15.0)	(17.4)	(15.0)	(14.8)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	0.0	(0.9)	0.0	(1.1)	(1.1)
Cumulative Effect of New Accounting Principle in Period of Adoption			0.0
Reclassification of Stranded Amounts in AOCI to Retained Earnings	0.0		0.0
AOCI, Net of Tax, After Cumulative Effect Adjustment	0.0		0.0
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment	0.0	0.6	0.0	0.8
Other comprehensive (income) loss attributable to NCI	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0	0.6	0.0	0.8
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
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.0	0.6	0.0	0.8
After tax total accumulated other comprehensive income, ending balance	0.0	(0.3)	0.0	(0.3)	0.0
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	6.6	2.1	2.0	4.3	4.3
Cumulative Effect of New Accounting Principle in Period of Adoption			0.0
Reclassification of Stranded Amounts in AOCI to Retained Earnings	(3.4)		(3.4)
AOCI, Net of Tax, After Cumulative Effect Adjustment	3.2		(1.4)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0.0	0.0	0.0	0.0
Forecasted transactions, after tax	0.0	0.0	0.0	0.0
Foreign currency translation adjustment	0.0	0.0	0.0	0.0
Other comprehensive (income) loss attributable to NCI	0.4	(0.7)	5.0	(2.9)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.4	(0.7)	5.0	(2.9)
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
Total reclassification adjustment for amounts realized in net income, after tax	0.0	0.0	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.4	(0.7)	5.0	(2.9)
After tax total accumulated other comprehensive income, ending balance	$ 3.6	$ 1.4	$ 3.6	$ 1.4	$ 2.0