Summary of Statutory Net Income (Loss) and Capital and Surplus (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
NIC and NSIC
Statutory Accounting Practices [Line Items]
Net income	$ 60.8	$ 75.7	$ 59.4
Capital and surplus	949.1	893.9	804.1
Syndicate
Statutory Accounting Practices [Line Items]
Net income	41.0	33.8	21.0
Capital and surplus	$ 177.5	$ 140.1	$ 124.2