Related Party Transactions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended									12 Months Ended
	Dec. 31, 2013 Tall Pines Insurance Company And Ewi Re Inc	Dec. 31, 2012 Tall Pines Insurance Company And Ewi Re Inc	Dec. 31, 2011 Tall Pines Insurance Company And Ewi Re Inc	Dec. 31, 2013 NL	Dec. 31, 2012 NL	Dec. 31, 2011 NL	Dec. 31, 2012 NL Unsecured Revolving Demand Promissory	Feb. 28, 2010 NL Unsecured Revolving Demand Promissory	Feb. 28, 2010 NL Unsecured Revolving Demand Promissory December 31, 2015	Dec. 31, 2012 NL Maximum Unsecured Revolving Demand Promissory	Dec. 31, 2012 NL Minimum Unsecured Revolving Demand Promissory	Dec. 31, 2013 Contran Corporation Intercorporate Services Agreements Fees	Dec. 31, 2012 Contran Corporation Intercorporate Services Agreements Fees	Dec. 31, 2011 Contran Corporation Intercorporate Services Agreements Fees	Dec. 31, 2013 Contran Corporation 2014 Intercorporate Services Agreements Fees Scenario Forecast
Related Party Transaction [Line Items]
Maximum loan amount							$ 40.0	$ 8.0
Interest rate on loans repayment									0.75%
Loans outstanding amount				0	0	0
Maturity date										Dec. 31, 2015	Mar. 31, 2015
Related party transaction expense	$ 1.0	$ 1.2	$ 1.3									$ 3.9	$ 3.7	$ 3.4	$ 3.0