Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (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
Segment Reporting Information [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
Net investment income									151,100,000	133,100,000	140,000,000.0
Fee and other income									9,100,000	9,000,000.0	22,500,000
Net realized investment gains (losses)									80,000,000.0	(72,000,000.0)	39,300,000
Total revenue									1,969,700,000	1,801,800,000	1,774,100,000
Foreign currency exchange gains									9,600,000	100,000	(6,300,000)
Other corporate expenses		(3,700,000)	(7,500,000)	(500,000)					(37,600,000)	0	0
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
U.S. Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									1,119,700,000	1,078,900,000	936,600,000
Net investment income									100,000,000.0	82,900,000	87,200,000
International Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									609,600,000	652,500,000	635,800,000
Net investment income									44,200,000	32,900,000	32,700,000
Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									200,000	300,000	(100,000)
Net investment income									5,700,000	8,100,000	9,300,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Net income (loss) before income taxes									(7,700,000)	185,000,000.0	64,800,000
Operating Segments [Member] | U.S. Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									1,119,700,000	1,078,900,000	936,600,000
Net investment income									100,000,000.0	82,900,000	87,200,000
Net income (loss) before income taxes									139,100,000	161,400,000	169,400,000
Operating Segments [Member] | International Operations [Member]
Segment Reporting Information [Line Items]
Earned premiums									609,600,000	652,500,000	635,800,000
Net investment income									44,200,000	32,900,000	32,700,000
Net income (loss) before income taxes									(137,000,000.0)	32,900,000	(86,700,000)
Operating Segments [Member] | Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									200,000	300,000	(100,000)
Net investment income									5,700,000	8,100,000	9,300,000
Net income (loss) before income taxes									(9,800,000)	(9,300,000)	(17,900,000)
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income									1,200,000	9,200,000	10,800,000
Net income (loss) before income taxes									$ (44,100,000)	$ (45,400,000)	$ (57,900,000)