Schedule of Additional Information about Significant Unobservable Inputs Used in Fair Valuations of Level Three (Detail) (USD $) In Millions, unless otherwise specified							6 Months Ended				6 Months Ended									6 Months Ended													6 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Jun. 30, 2012 Fair Value, Inputs, Level 3	Jun. 30, 2012 Fair Value, Inputs, Level 3 Municipal	Jun. 30, 2012 Fair Value, Inputs, Level 3 Corporate	Jun. 30, 2012 Fair Value, Inputs, Level 3 Commercial Mortgage-Backed	Jun. 30, 2012 Fair Value, Inputs, Level 3 Asset-Backed	Jun. 30, 2012 Discount For Small Issue Size Fair Value, Inputs, Level 3 Discounted Cash Flow Municipal	Jun. 30, 2012 Discount For Small Issue Size Fair Value, Inputs, Level 3 Discounted Cash Flow Corporate	Jun. 30, 2012 Discount For Small Issue Size Fair Value, Inputs, Level 3 Discounted Cash Flow Residential Mortgage-Backed	Jun. 30, 2012 Discount For Small Issue Size Fair Value, Inputs, Level 3 Discounted Cash Flow Commercial Mortgage-Backed	Jun. 30, 2012 Discount For Small Issue Size Fair Value, Inputs, Level 3 Discounted Cash Flow Asset-Backed	Jun. 30, 2012 Discount For Long Maturity Fair Value, Inputs, Level 3 Discounted Cash Flow Municipal	Jun. 30, 2012 Discount For Long Maturity Fair Value, Inputs, Level 3 Discounted Cash Flow Corporate	Jun. 30, 2012 Discount For Long Maturity Fair Value, Inputs, Level 3 Discounted Cash Flow Asset-Backed	Jun. 30, 2012 Discount For Above Market Coupon Fair Value, Inputs, Level 3 Discounted Cash Flow Municipal	Jun. 30, 2012 Discount For Above Market Coupon Fair Value, Inputs, Level 3 Discounted Cash Flow Corporate	Jun. 30, 2012 Discount For Above Market Coupon Fair Value, Inputs, Level 3 Discounted Cash Flow Commercial Mortgage-Backed	Jun. 30, 2012 Discount For Above Market Coupon Fair Value, Inputs, Level 3 Discounted Cash Flow Asset-Backed	Jun. 30, 2012 Discount For Credit Stress Fair Value, Inputs, Level 3 Discounted Cash Flow Corporate	Jun. 30, 2012 Discount For Credit Stress Fair Value, Inputs, Level 3 Discounted Cash Flow Commercial Mortgage-Backed	Jun. 30, 2012 Discount For Lease Structure Fair Value, Inputs, Level 3 Asset-Backed	Jun. 30, 2012 Equity Securities	Mar. 31, 2012 Equity Securities	Dec. 31, 2011 Equity Securities	Jun. 30, 2011 Equity Securities	Mar. 31, 2011 Equity Securities	Dec. 31, 2010 Equity Securities	Jun. 30, 2012 Equity Securities Fair Value, Inputs, Level 3
Fair Value [Line Items]
Fair value measurement market multiples																																	1.0
Fair value measurement weighted average market multiples																																	1.0
Fair Value	$ 99.0	$ 96.6	$ 98.9	$ 74.9	$ 72.4	$ 67.5		$ 16.9	$ 27.2	$ 4.3				$ 0.6					$ 24.2								$ 25.6	$ 21.4	$ 23.4	$ 2.9	$ 2.9	$ 2.9	$ 25.5
Discount rate							0.30%				0.30%			0.50%	0.50%		0.50%	0.50%				0.50%			1.00%
Discount rate, minimum												1.00%	0.30%			0.50%				0.30%	0.30%		0.30%	2.50%		0.30%
Discount rate, maximum												4.00%	3.00%			0.80%				1.00%	1.00%		2.00%	3.00%		1.00%
Weighted average rate							0.30%				0.30%	3.10%	0.60%	0.50%	0.50%	0.70%	0.50%	0.50%		0.50%	0.70%	0.50%	0.60%	2.50%	1.00%	0.40%