Run-Off Lines - Total Gross Reserves for Asbestos Exposure (Detail) - Asbestos and Environmental Claims [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	$ 21.4	$ 15.4	$ 10.7
Total asbestos reserves	48.6	41.9	42.8
Case Reserves [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	2.1	2.8	2.0
ULAE [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	0.5	0.5	0.5
IBNR [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total direct written reserves	18.8	12.1	8.2
Assumed domestic [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	24.3	21.8	26.7
Assumed domestic [Member] | Case Reserves [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	9.8	10.5	12.2
Assumed domestic [Member] | ULAE [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	0.8	0.8	0.8
Assumed domestic [Member] | IBNR [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	13.7	10.5	13.7
Assumed London [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	2.9	4.7	5.4
Assumed London [Member] | Case Reserves [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	2.3	3.3	4.0
Assumed London [Member] | ULAE [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	0.0	0.0	0.0
Assumed London [Member] | IBNR [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Total assumed reserves	$ 0.6	$ 1.4	$ 1.4