Schedule of Additional Information about Significant Unobservable Inputs Used in Fair Valuations of Level Three (Detail) (USD $) In Millions, unless otherwise specified					3 Months Ended										3 Months Ended																				3 Months Ended																										3 Months Ended								3 Months Ended																															3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2013 Equity Securities Fair Value, Inputs, Level 3	Dec. 31, 2012 Equity Securities Fair Value, Inputs, Level 3	Mar. 31, 2013 Other securities	Dec. 31, 2012 Other securities	Mar. 31, 2013 Other securities Minimum	Dec. 31, 2012 Other securities Minimum	Mar. 31, 2013 Other securities Maximum	Dec. 31, 2012 Other securities Maximum	Mar. 31, 2013 Municipal Fair Value, Inputs, Level 3	Dec. 31, 2012 Municipal Fair Value, Inputs, Level 3	Mar. 31, 2013 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Municipal Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Municipal Discount For Above Market Coupon 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	Mar. 31, 2013 Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Municipal Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Municipal Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Corporate Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Fair Value, Inputs, Level 3	Mar. 31, 2013 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Corporate Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Corporate Discount For Above Market Coupon 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	Mar. 31, 2013 Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Corporate Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Corporate Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Corporate Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Residential mortgage-backed Fair Value, Inputs, Level 3	Dec. 31, 2012 Residential mortgage-backed Fair Value, Inputs, Level 3	Mar. 31, 2013 Residential mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Residential mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Residential mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Residential mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Residential mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Residential mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Commercial mortgage-backed Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial mortgage-backed Fair Value, Inputs, Level 3	Mar. 31, 2013 Commercial mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3	Mar. 31, 2013 Commercial mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Commercial mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Commercial mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Commercial mortgage-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Commercial mortgage-backed Discount For Above Market Coupon 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	Mar. 31, 2013 Commercial mortgage-backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Commercial mortgage-backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Commercial mortgage-backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Commercial mortgage-backed Discount For Above Market Coupon Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Commercial mortgage-backed Discount For Long Maturity 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	Mar. 31, 2013 Commercial mortgage-backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Commercial mortgage-backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Commercial mortgage-backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Commercial mortgage-backed Discount For Long Maturity Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Commercial mortgage-backed Discount For Credit Stress 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	Mar. 31, 2013 Commercial mortgage-backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Commercial mortgage-backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Commercial mortgage-backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Commercial mortgage-backed Discount For Credit Stress Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum	Mar. 31, 2013 Commercial mortgage-backed Discount For Lease Structure Fair Value, Inputs, Level 3	Dec. 31, 2012 Commercial mortgage-backed Discount For Lease Structure Fair Value, Inputs, Level 3	Mar. 31, 2013 Commercial mortgage-backed Discount For Lease Structure Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Commercial mortgage-backed Discount For Lease Structure Fair Value, Inputs, Level 3 Minimum	Mar. 31, 2013 Commercial mortgage-backed Discount For Lease Structure Fair Value, Inputs, Level 3 Maximum	Dec. 31, 2012 Commercial mortgage-backed Discount For Lease Structure Fair Value, Inputs, Level 3 Maximum	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 Asset-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Minimum	Dec. 31, 2012 Asset-backed Discount For Small Issue Size Discounted Cash Flow Fair Value, Inputs, Level 3 Maximum
Fair Value [Line Items]
Fair value measurement market multiples					1	0.9
Fair value measurement weighted average market multiples					1	0.9
Fair Value	$ 105.6	$ 102.7	$ 100.2	$ 102.5	$ 24.9	$ 24.3	$ 3.6	$ 3.6					$ 28.8	$ 19.4																			$ 22.6	$ 26.4																									$ 0.6	$ 0.7							$ 24.8	$ 26.7																															$ 1.5
Weighted average rate							18.00%	18.00%	18.00%	18.00%	18.00%	18.00%			2.40%	3.10%	1.00%	1.00%	4.00%	4.00%	0.50%	0.50%	0.30%	0.30%	1.00%	1.00%	0.30%	0.50%	0.10%	0.50%	0.60%	0.50%			0.60%	0.50%	0.30%	0.30%	3.00%	3.00%	0.70%	0.70%	0.30%	0.30%	1.00%	1.00%	0.10%	0.50%	0.10%	0.50%	0.10%	0.50%	3.00%	1.10%	3.00%	1.00%	3.00%	3.00%			0.50%	0.50%	0.50%	0.50%	0.50%	0.50%			0.50%	0.50%	0.50%	0.50%	0.50%	0.50%	0.40%	0.40%	0.30%	0.30%	0.80%	0.80%	0.40%	0.70%	0.20%	0.50%	0.50%	0.80%	1.00%	1.00%	1.00%	1.00%	1.00%	1.00%	0.30%	0.30%	0.30%	0.30%	0.30%	0.30%		1.60%	0.70%	2.00%