Underwriting, Acquisition and Insurance Expenses - Underwriting, Acquisition and Insurance Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Other Income And Expenses [Abstract]
Commissions	$ 67.2	$ 63.3	$ 125.9	$ 118.2
General expenses	79.7	74.1	152.6	142.9
Premium taxes, boards and bureaus	0.0	5.7	0.6	12.3
Underwriting, acquisition and insurance expenses, total	146.9	143.1	279.1	273.4
Net deferral of policy acquisition costs	(7.4)	(6.3)	(10.0)	(7.9)
Total underwriting, acquisition and insurance expenses	$ 139.5	$ 136.8	$ 269.1	$ 265.5