Income Taxes - Schedule of Net Deferred Tax Liability Comprises Tax Effect Related to Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Losses and loss adjustment expense reserve discounting	$ 22.8	$ 19.3
Unearned premiums	25.3	23.2
Net operating loss carryforwards	30.6	31.6
Investment in limited partnership interests	7.1	12.0
Unrealized losses on fixed maturities and other investment securities	0.0	9.0
Investment	2.4	1.3
Right of use assets	14.5
Accrued bonus	2.2	4.5
Stock option expense	1.1	1.7
United Kingdom underwriting results	5.9	0.0
Other	5.7	6.3
Deferred tax assets, gross	117.6	108.9
Deferred tax liabilities:
Unrealized gains on equity securities	(3.0)	(13.1)
Unrealized gains on fixed maturities and other investment securities	(5.3)	0.0
Unrealized gains on limited partnership interests	(15.6)	(17.3)
Depreciable fixed assets	(22.4)	(27.0)
Deferred acquisition costs	(18.6)	(18.1)
Lease liability	(14.0)
TCJA reserve transitional liability	(3.2)	(4.5)
United Kingdom underwriting results	0.0	(0.6)
Other	(0.6)	(4.6)
Deferred tax liabilities, gross	(82.7)	(85.2)
Deferred tax assets, net before valuation allowance	34.9	23.7
Valuation allowance	(28.8)	(29.9)
Deferred tax asset (liabilities), net		(6.2)
Net deferred tax assets (liabilities) - Other jurisdictions	6.1
Foreign Tax Authority [Member]
Deferred tax liabilities:
Deferred tax asset (liabilities), net		(0.6)
Net deferred tax assets (liabilities) - Other jurisdictions	6.1
Domestic Tax Authority [Member]
Deferred tax liabilities:
Deferred tax asset (liabilities), net		$ (5.6)
Net deferred tax assets (liabilities) - Other jurisdictions	$ 0.0