Summary of Statutory Net Income (Loss) and Capital and Surplus (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
NIC and NSIC
Statutory Accounting Practices [Line Items]
Net income	$ 84.2	$ 60.8	$ 75.7
Capital and surplus	1,027.3	949.1	893.9
Syndicate
Statutory Accounting Practices [Line Items]
Net income	46.3	41.0	33.8
Capital and surplus	$ 180.7	$ 177.5	$ 140.1