10. Fair Value of Financial Instruments (Details 4) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
U.S. government and agency [Member]
Level 3 Assets:
Fair Value, Total assets, Balance	$ 25.9	$ 31.7	$ 24.0
Valuation Techniques	Spread matrix
Unobservable Input	Yield
Range (Weighted Average)	1.46% - 3.49% (2.57%)
State and political subdivision [Member]
Level 3 Assets:
Fair Value, Total assets, Balance	58.0	48.7	13.3
Valuation Techniques	Spread matrix
Unobservable Input	Yield
Range (Weighted Average)	1.94% - 3.53% (2.97%)
State and political subdivision [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Yield
Range (Weighted Average)	3.28%
Corporate [Member]
Level 3 Assets:
Fair Value, Total assets, Balance	649.6	513.8	337.3
Valuation Techniques	Spread matrix
Unobservable Input	Yield
Range (Weighted Average)	1.47% - 6.33% (3.00%)
Corporate [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Yield
Range (Weighted Average)	1.48% - 5.94% (3.30%)
CDO/CLO [Member]
Level 3 Assets:
Fair Value, Total assets, Balance	3.4	62.5	59.6
Valuation Techniques	Discounted cash flow
Unobservable Input	Prepayment rate
Range (Weighted Average)	20% (CLOs)
CDO/CLO [Member]
Level 3 Assets:
Unobservable Input	Recovery rate
Range (Weighted Average)	65% (Loans), 35% (High yield bonds),
CDO/CLO [Member]
Level 3 Assets:
Unobservable Input	Recovery rate
Range (Weighted Average)	45% (Investment grade bonds)
CDO/CLO [member]
Level 3 Assets:
Unobservable Input	Reinvestment spread
Range (Weighted Average)	3 mo LIBOR + 400bps (CLOs)
Other asset-backed [Member]
Level 3 Assets:
Fair Value, Total assets, Balance	2.8	81.2	57.7
Valuation Techniques	Discounted cash flow
Unobservable Input	Yield
Range (Weighted Average)	2.61% - 9.50% (4.97%)
Other asset-backed [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Prepayment rate
Range (Weighted Average)	2% (CLOs)
Other asset-backed [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Default rate
Range (Weighted Average)	2.53% for 48 mos then .33% thereafter
Other asset-backed [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Recovery rate
Range (Weighted Average)	65% (Loans)
Other asset-backed [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Reinvestment spread
Range (Weighted Average)	3mo LIBOR + 400bps (CLOs)
Fair value option investments [Member]
Level 3 Assets:
Fair Value, Total assets, Balance	$ 22.7	$ 27.5	$ 13.8
Valuation Techniques	Discounted cash flow
Unobservable Input	Default rate
Range (Weighted Average)	0.24%
Fair value option investments [Member]
Level 3 Assets:
Valuation Techniques	Discounted cash flow
Unobservable Input	Recovery rate
Range (Weighted Average)	45%