Income Taxes - Schedule of Net Deferred Tax Liability Comprises Tax Effect Related to Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Deferred tax assets:
Losses and loss adjustment expense reserve discounting	$ 35.4	$ 29.2
Unearned premiums	27.0	25.9
Net operating loss carryforwards	31.6	27.9
Investment in limited partnership interests	21.1	7.8
Unrealized losses on equity securities	8.6	0.0
Investments	0.0	2.0
Right of use assets	13.5	12.7
Accrued bonus	5.8	6.3
Stock option expense	1.1	0.7
United Kingdom underwriting results	28.1	21.9
Other	10.6	9.6
Deferred tax assets, gross	182.8	144.0
Deferred tax liabilities:
Unrealized gains on equity securities	0.0	(5.7)
Unrealized gains on fixed maturities and other investment securities	(4.8)	(22.3)
Unrealized gains on limited partnership interests	(26.3)	(14.7)
Investments	(5.1)	0.0
Depreciable fixed assets	(7.9)	(20.5)
Deferred acquisition costs	(21.3)	(20.4)
Lease liability	(12.2)	(11.7)
TCJA reserve transitional liability	(2.1)	(2.7)
Other	(1.6)	(0.7)
Deferred tax liabilities, gross	(81.3)	(98.7)
Deferred tax assets, net before valuation allowance	101.5	45.3
Valuation allowance	(27.9)	(28.6)
Net deferred assets	73.6	16.7
Foreign Tax Authority
Deferred tax liabilities:
Net deferred assets	35.0	21.4
Domestic Tax Authority
Deferred tax liabilities:
Net deferred assets	$ 38.6
Net deferred liabilities		$ (4.7)