Summary of Statutory Net Income (Loss) and Capital and Surplus (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NIC and NSIC
Statutory Accounting Practices [Line Items]
Net Income	$ 79.3	$ 84.2	$ 60.8
Capital and Surplus	1,056.6	1,027.3	949.1
Syndicate
Statutory Accounting Practices [Line Items]
Net Income	(23.4)	46.3	41.0
Capital and Surplus	168.7	180.7	$ 177.5
NIIC
Statutory Accounting Practices [Line Items]
Net Income	(8.4)	0.4
Capital and Surplus	$ 62.9	$ 70.7