Underwriting, Acquisition and Insurance Expenses & Other Corporate Expenses (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Underwriting Acquisition And Insurance Expenses [Abstract]
Commissions									$ 241,300,000	$ 280,400,000	$ 255,900,000
General expenses									379,100,000	351,700,000	359,100,000
Premium taxes, boards and bureaus									33,000,000.0	32,500,000	32,300,000
Underwriting, acquisition and insurance expenses, total									653,400,000	664,600,000	647,300,000
Net deferral of policy acquisition costs									12,400,000	(9,900,000)	(11,900,000)
Total underwriting, acquisition and insurance expenses	$ 180,200,000	$ 164,000,000.0	$ 161,400,000	$ 160,200,000	$ 169,700,000	$ 168,000,000.0	$ 156,800,000	$ 160,200,000	665,800,000	654,700,000	635,400,000
Other corporate expenses		$ 3,700,000	$ 7,500,000	$ 500,000					$ 37,600,000	$ 0	$ 0