Schedule of Additional Information about Significant Unobservable Inputs Used in Fair Valuations of Level Three (Detail) (USD $) In Millions, unless otherwise specified				12 Months Ended			12 Months Ended				12 Months Ended					12 Months Ended		12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Equity Securities Fair Value, Inputs, Level 3	Dec. 31, 2012 Other securities	Dec. 31, 2012 Municipal Fair Value, Inputs, Level 3	Dec. 31, 2012 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Residential Mortgage-Backed Fair Value, Inputs, Level 3	Dec. 31, 2012 Residential Mortgage-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial Mortgage-Backed Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial Mortgage-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial Mortgage-Backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial Mortgage-Backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial Mortgage-Backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial Mortgage-Backed Discount For Lease Structure Fair Value, Inputs, Level 3	Dec. 31, 2012 Asset-Backed Fair Value, Inputs, Level 3	Dec. 31, 2012 Asset-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Commercial Mortgage-Backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Commercial Mortgage-Backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Minimum Asset-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Equity Securities Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Other securities	Dec. 31, 2012 Maximum Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Residential Mortgage-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Commercial Mortgage-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Commercial Mortgage-Backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Commercial Mortgage-Backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Commercial Mortgage-Backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Commercial Mortgage-Backed Discount For Lease Structure Fair Value, Inputs, Level 3	Dec. 31, 2012 Maximum Asset-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3
Fair Value [Line Items]
Fair Value	$ 102.7	$ 102.5	$ 70.9	$ 24.3	$ 3.6	$ 19.4				$ 26.4					$ 0.7		$ 26.7						$ 1.5
Weighted average rate				0.90%	18.00%		3.10%	0.50%	0.50%		0.50%	0.50%	0.70%	1.10%		0.50%		0.50%	0.70%	0.40%	1.00%	0.30%		1.60%	1.00%	0.30%	0.30%	0.30%	1.00%	0.50%	0.30%	0.70%	0.90%	18.00%	4.00%	0.50%	1.00%	3.00%	0.50%	1.00%	3.00%	0.50%	0.50%	0.80%	0.80%	1.00%	0.30%	2.00%