Income Taxes - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Deferred Tax Assets [Line Items]
U.S. statutory federal income tax rate	35.00%
Deferred tax assets, U.S. net operating loss carryforwards	$ 42.1
Deferred tax assets, foreign net operating loss carryforwards	22.4
Deferred tax asset, alternative minimum tax carryforward	110.3
Deferred tax assets, Foreign tax credit carryforwards	12.8
Valuation Allowance	2.9	35.9	35.9	91.5
Unrealized gains in our investment portfolio		69.6	98.4
Valuation allowance, deferred tax asset, reduction (increase) in amount			55.6
Increase in accumulated other comprehensive income due to valuation allowance decrease			28.8
Income recognized in continuing operations related to non-segment income	107.7
Tax benefit recorded as income from discontinued operations		2.9
Net realized investment gains	33.5	23.6	28.1
Tax credit carryforwards expired			7.6
Foreign income permanently reinvested	20.3	10.0	4.2
Estimated taxes payable on undistributed earnings	13.0
Deferred deductions	4.8
Accrued interest	1.2	0.4
Non Reportable Segment
Deferred Tax Assets [Line Items]
Income recognized in continuing operations related to non-segment income	17.5	14.3	9.7
Valuation Allowance, Capital Loss Carryforwards Realized due to Unrealized Gains
Deferred Tax Assets [Line Items]
Valuation Allowance		24.4	29.0
Valuation Allowance, Unrealized Appreciation
Deferred Tax Assets [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount		25.3	21.9
Valuation Allowance, Other
Deferred Tax Assets [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			0.2
Increase in income from continuing operations due to valuation allowance decrease		7.7
Valuation Allowance, Capital Loss Carryforwards Realized due to Net Realized Gain
Deferred Tax Assets [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			7.5
Valuation Allowance, Portion Utilized before Expiration
Deferred Tax Assets [Line Items]
Valuation allowance, deferred tax asset, reduction (increase) in amount			(2.6)
Foreign Tax Credit Carryforwards
Deferred Tax Assets [Line Items]
Foreign tax credit carryforwards expiration date	Beginning in 2022
Domestic Tax Authority
Deferred Tax Assets [Line Items]
Pre-tax operating loss carryforward	120.3
Operating loss expiration date	Beginning in 2031
U.K.
Deferred Tax Assets [Line Items]
Pre-tax operating loss carryforward	$ 66.2
Operating loss expiration date	No expiration date