Schedule V Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Real Estate reserve (deducted from other long-term investments)
Valuation and Qualifying Accounts Disclosure
Balance at Beginning of Year		$ 4.2	$ 1.2	$ 0.3
Additions Charged to Costs and Expenses		0.0	3.0	1.2
Additions Charged to Other Accounts	[1]	0.0	0.0	0.0
Deductions	[2]	0.0	0.0	0.3
Balance at End of Year		4.2	4.2	1.2
Allowance for doubtful accounts (deducted from accounts and premiums receivable)
Valuation and Qualifying Accounts Disclosure
Balance at Beginning of Year		6.2	6.2	6.4
Additions Charged to Costs and Expenses		2.3	3.1	1.7
Additions Charged to Other Accounts	[1]	0.1	0.0	0.0
Deductions	[2]	2.2	3.1	1.9
Balance at End of Year		$ 6.4	$ 6.2	$ 6.2

[1] Additions charged to other accounts are comprised of amounts related to fluctuations in the foreign currency exchange rate.

[2] Deductions include amounts deemed to reduce exposure of probable losses, amounts deemed uncollectible, and amounts related to fluctuations in the foreign currency exchange rate.