Unaudited Quarterly Financial Data (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	Feb. 20, 2018
Class of Stock [Line Items]
Earned premiums	$ 425,800,000	$ 451,500,000	$ 431,700,000	$ 420,500,000	$ 452,400,000	$ 446,900,000	$ 417,700,000	$ 414,700,000	$ 1,729,500,000	$ 1,731,700,000	$ 1,572,300,000
Losses and loss adjustment expenses	359,200,000	338,800,000	284,800,000	237,900,000	280,600,000	277,500,000	245,500,000	237,200,000	1,220,700,000	1,040,800,000	1,050,200,000
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
Underwriting income (loss)	(113,600,000)	(51,300,000)	(14,500,000)	22,400,000	2,100,000	1,400,000	15,400,000	17,300,000	(157,000,000.0)	36,200,000
Net income (loss) before income taxes	(103,300,000)	(26,200,000)	29,600,000	100,100,000	(59,700,000)	45,300,000	57,100,000	25,000,000.0	200,000	67,700,000	39,900,000
Net income (loss)	$ (103,300,000)	$ (25,100,000)	$ 28,800,000	$ 91,200,000	$ (43,600,000)	$ 40,600,000	$ 41,800,000	$ 24,800,000	$ (8,400,000)	$ 63,600,000	$ 50,300,000
Net income (loss) per common share:
Basic (in dollars per share)	$ (3.01)	$ (0.73)	$ 0.84	$ 2.68	$ (1.29)	$ 1.20	$ 1.23	$ 0.73	$ (0.25)	$ 1.87	$ 1.46
Diluted (in dollars per share)	(3.01)	(0.73)	0.83	2.63	(1.29)	1.17	1.20	0.71	(0.25)	1.83	1.42
Dividends per common share (in dollars per shares)	$ 0.31	$ 0.31	$ 0.31	$ 0.31	$ 0.27	$ 0.27	$ 0.27	$ 0.27	$ 1.24	$ 1.08	$ 0.94
Other corporate expenses		$ 3,700,000	$ 7,500,000	$ 500,000					$ 37,600,000	$ 0	$ 0
Stock dividend declared, percent					15.00%					15.00%
Stock Dividend [Member]
Net income (loss) per common share:
Stock dividend declared, percent												15.00%