Business and Significant Accounting Policies - Additional Information (Detail) $ in Millions					12 Months Ended
	Feb. 20, 2018 shares	Feb. 06, 2017 USD ($)	May 03, 2016 shares	Feb. 17, 2015 shares	Dec. 31, 2018	Dec. 31, 2017 USD ($) Trust Institution	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business And Significant Accounting Policies [Line Items]
Percentage of contractual participation						100.00%
Number of Statutory Trusts | Trust						12
Number of Charitable Foundations | Institution						2
Stock dividend declared							10.00%	10.00%
Short-term investments, maturity term						1 year
Percentage of investment portfolio in fair value from pricing service or broker						1.00%
Allowance for doubtful accounts						$ 0.6	$ 2.7
Reinsurance recoverables, allowance for doubtful accounts						2.1	2.1
Estimated liability for return of premiums						7.9	5.7
Estimated amount of premiums receivables due						$ 0.1	0.1
Weighted average useful life						8 years 6 months
Amortization expense						$ 5.9	5.5	$ 7.5
Estimated amortization expense for 2018						3.5
Estimated amortization expense for 2019						1.7
Estimated amortization expense for 2020						1.3
Estimated amortization expense for 2021						1.3
Estimated amortization expense for 2022						0.1
Accumulated depreciation for property and equipment						124.1	101.6
Net book value of property and equipment						151.0	142.2
Depreciation expense						24.1	20.9	17.5
Foreign currency translation adjustment						$ 19.0	17.6
US federal tax rate						35.00%
Income taxes paid						$ 16.5	16.6	10.9
Income taxes recovered						$ 2.5	$ 0.5	$ 11.7
Scenario Forecast [Member]
Business And Significant Accounting Policies [Line Items]
US federal tax rate					21.00%
Minimum [Member]
Business And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						3 years
Maximum [Member]
Business And Significant Accounting Policies [Line Items]
Property, Plant and Equipment, Useful Life						39 years
Distribution Network [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life						8 years 10 months 24 days
Additional Lloyd's Capacity [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life						5 years
Other [Member]
Business And Significant Accounting Policies [Line Items]
Weighted average useful life						6 years 4 months 24 days
Stock Dividend [Member]
Business And Significant Accounting Policies [Line Items]
Stock dividend declared			10.00%	10.00%
Dividends declaration date			May 03, 2016	Feb. 17, 2015
Dividends payable date			Jun. 15, 2016	Mar. 16, 2015
Dividend payable, date of record			Jun. 01, 2016	Mar. 02, 2015
Additional stock issued as dividend | shares			2,735,542	2,554,506
Subsequent Event [Member]
Business And Significant Accounting Policies [Line Items]
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 23, 2018
Dividend payable, date of record	Mar. 07, 2018
Subsequent Event [Member] | Stock Dividend [Member]
Business And Significant Accounting Policies [Line Items]
Stock dividend declared	15.00%
Dividends declaration date	Feb. 20, 2018
Dividends payable date	Mar. 21, 2018
Dividend payable, date of record	Mar. 07, 2018
Additional stock issued as dividend | shares	4,400,000
Maybrooke [Member]
Business And Significant Accounting Policies [Line Items]
Business acquisition effective date						Feb. 06, 2017
Intangible assets acquired		$ 35.1
Amortization expense						$ 1.2
Maybrooke [Member] | Distribution Network [Member]
Business And Significant Accounting Policies [Line Items]
Intangible assets acquired		$ 4.7
Weighted average useful life		5 years