Income Taxes - Additional Disclosure (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended							12 Months Ended										24 Months Ended				12 Months Ended					1 Months Ended	12 Months Ended
	Dec. 31, 2012	Jun. 30, 2012	Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Jul. 31, 2010	Jan. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Non Reportable Segment	Dec. 31, 2012 Foreign Tax Credit Carryforwards	Dec. 31, 2012 Domestic Tax Authority	Dec. 31, 2012 U.K.	Dec. 31, 2010 Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains	Dec. 31, 2009 Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains	Dec. 31, 2010 Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains	Jun. 30, 2012 Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains	Dec. 31, 2011 Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains	Sep. 30, 2011 Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains	Dec. 31, 2012 Valuation Allowance, Unrealized Appreciation	Dec. 31, 2011 Valuation Allowance, Unrealized Appreciation	Dec. 31, 2011 Valuation Allowance, Capital Loss Carryforwards Realized due to Net Realized Gain	Dec. 31, 2011 Valuation Allowance, Portion Utilized before Expiration	Dec. 31, 2010 Valuation Allowance, Portion Utilized before Expiration	Sep. 30, 2011 Valuation Allowance, Other	Dec. 31, 2012 Valuation Allowance, Other	Dec. 31, 2011 Valuation Allowance, Other	Dec. 31, 2010 Valuation Allowance, Other	Dec. 31, 2010 Valuation Allowance, Portion of Loss Cannot be Utilized
Deferred Tax Assets [Line Items]
U.S. statutory federal income tax rate								35.00%
Deferred tax assets, U.S. net operating loss carryforwards	$ 58.4							$ 58.4
Deferred tax assets, foreign net operating loss carryforwards	16.7							16.7
Pre-tax operating loss carryforward														166.8	70.7
Operating loss expiration date														Beginning in 2031	No expiration date
Deferred tax asset, alternative minimum tax carryforward	112.2							112.2
Deferred tax assets, Foreign tax credit carryforwards	14.6							14.6
Foreign tax credit carryforwards expiration date													Beginning in 2021
Unrealized gains in our investment portfolio		69.6	98.4	120.8	31.1	37.1	98.4
Valuation allowance	35.9			91.5				35.9	35.9	91.5	195.6								24.4	29.0	29.0
Income recognized in continuing operations related to non-segment income	12.6	0.3							9.5			1.4
Increase in accumulated other comprehensive income due to valuation allowance decrease		24.1	28.8					28.8	91.4	91.4
Valuation allowance, deferred tax asset, reduction (increase) in amount									55.6	104.1						66.2	34.4	100.6				25.3	21.9	7.5	(2.6)	47.4	0.2		0.2	1.1	(20.3)
Tax benefit recorded as income from discontinued operations								2.9	2.1
Increase in income from continuing operations due to valuation allowance decrease										3.2	6.0																	7.7
Net realized investment gains								23.6	28.1	29.7
Tax credit carryforwards expired									7.6	135.5
Foreign income permanently reinvested								10.0	4.2
Estimated taxes payable on undistributed earnings	5.0							5.0
Liability at beginning of year, net							0.8	0.8	0.8	0.8
Tax benefit receivable										3.6
Decrease in accrued interest								0.7
Accrued interest	$ 0.4							$ 0.4	$ 0.3