Schedule of Additional Information about Significant Unobservable Inputs Used in Fair Valuations of Level Three (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified							9 Months Ended		9 Months Ended				9 Months Ended					9 Months Ended		9 Months Ended						9 Months Ended
	Sep. 30, 2012	Jun. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2012 Equity Securities Fair Value, Inputs, Level 3	Sep. 30, 2012 Municipal Fair Value, Inputs, Level 3	Sep. 30, 2012 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Corporate Fair Value, Inputs, Level 3	Sep. 30, 2012 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Residential Mortgage-Backed Fair Value, Inputs, Level 3	Sep. 30, 2012 Residential Mortgage-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Commercial Mortgage-Backed Fair Value, Inputs, Level 3	Sep. 30, 2012 Commercial Mortgage-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Commercial Mortgage-Backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Commercial Mortgage-Backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Commercial Mortgage-Backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Commercial Mortgage-Backed Discount For Lease Structure Fair Value, Inputs, Level 3	Sep. 30, 2012 Asset-Backed Fair Value, Inputs, Level 3	Sep. 30, 2012 Asset-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Commercial Mortgage-Backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Commercial Mortgage-Backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Minimum Asset-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Commercial Mortgage-Backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Commercial Mortgage-Backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3	Sep. 30, 2012 Maximum Asset-Backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3
Fair Value [Line Items]
Fair value measurement market multiples							$ 0.9
Fair value measurement weighted average market multiples							$ 0.9
Fair Value	$ 103.7	$ 102.6	$ 102.5	$ 106.7	$ 78.3	$ 70.9	$ 23.8	$ 19.6				$ 27.2					$ 0.7		$ 27.4						$ 1.1
Discount rate										0.50%				0.50%				0.50%		0.50%			1.00%	0.30%			1.00%	0.30%	0.30%	0.30%	2.50%	0.50%	0.30%	0.30%	4.00%	1.00%	3.00%	1.00%	3.00%	0.80%	1.00%	2.00%
Weighted average rate									3.10%	0.50%	0.50%		0.60%	0.50%	0.70%	2.50%		0.50%		0.50%	0.70%	0.40%	1.00%	0.30%		1.90%