Disclosures about Fair Value of Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Financial Instruments Owned and Pledged as Collateral [Line Items]
Premiums receivable	$ 672.6	$ 463.8	[1]
Reinsurance recoverables	1,519.6	1,385.6	[1]
Premiums receivable, allowance for doubtful accounts	2.9	2.7
Over 90 Days [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Premiums receivable	19.3	14.3
Reinsurance recoverables	12.2	11.2
Reinsurance recoverables, allowance for doubtful accounts	2.1	2.1
Premiums receivables, secured by collateral		0.1
Reinsurance recoverables, secured by collateral	$ 0.6	$ 0.6

[1] Derived from audited consolidated financial statements.