Segment Information - Revenue and Income (Loss) Before Income Taxes for Each Segment (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Earned premiums									$ 1,410.8	$ 1,371.9	$ 1,338.1
Net investment income									115.1	88.6	106.1
Fee and other income									24.5	22.2	20.7
Net realized investment and other gains									26.1	24.1	74.5
Total revenue	$ 405.4	$ 416.7	$ 384.3	$ 370.1	$ 373.4	$ 380.2	$ 377.2	$ 376.0	1,576.5	1,506.8	1,539.4
Income (loss) before income taxes	$ 47.0	$ 62.2	$ 39.6	$ 33.1	$ 44.2	$ 36.2	$ 34.7	$ 62.4	181.9	177.5	216.0
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Net investment income									3.2	8.0	3.1
Income (loss) before income taxes									(55.0)	(28.0)	(35.0)
Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									210.8	181.4	176.5
Excess and Surplus Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									485.3	471.2	444.6
Net investment income									45.2	32.3	43.2
Excess and Surplus Lines [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									88.4	81.4	103.4
Commercial Specialty [Member]
Segment Reporting Information [Line Items]
Earned premiums									364.2	344.2	332.5
Net investment income									26.7	19.9	25.6
Commercial Specialty [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									86.0	43.0	24.8
International Specialty [Member]
Segment Reporting Information [Line Items]
Earned premiums									154.5	146.4	144.8
Net investment income									16.8	11.4	11.3
International Specialty [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									38.2	29.0	23.8
Syndicate 1200 [Member]
Segment Reporting Information [Line Items]
Earned premiums									406.4	409.7	414.6
Net investment income									11.9	8.9	10.9
Syndicate 1200 [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									13.4	35.4	45.2
Run Off Lines [Member]
Segment Reporting Information [Line Items]
Earned premiums									0.4	0.4	1.6
Net investment income									11.3	8.1	12.0
Run Off Lines [Member] | Operating Segments [Members]
Segment Reporting Information [Line Items]
Income (loss) before income taxes									$ (15.2)	$ (7.4)	$ (20.7)