Level 3 Assets - Unobservable inputs (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012
Level 3 | U.S. government and agency
Fair Value, Option, Quantitative Disclosures [Line Items]
U.S. government and agency	$ 58.8	[1]	$ 25.9	[1]
Level 3 | State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
State and political subdivision	45.4	[1]	58.0	[1]
Level 3 | Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Corporate	874.1	[1]	649.6	[1]
Level 3 | CDO/CLO
Fair Value, Option, Quantitative Disclosures [Line Items]
CDO/CLO			3.4	[1]
Level 3 | Other asset-backed
Fair Value, Option, Quantitative Disclosures [Line Items]
Other asset-backed	11.3	[1]	2.8	[1]
Level 3 | Fair value investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value investments	$ 0.8	[1]	$ 0.8	[1]
Discounted cash flow | Other asset-backed
Fair Value, Option, Quantitative Disclosures [Line Items]
Prepayment rate	2.00%	[1]	2.00%	[1]
Discounted cash flow | Fair value investments
Fair Value, Option, Quantitative Disclosures [Line Items]
Default rate	0.25%	[1]	0.24%	[1]
Recovery rate	45.00%	[1]	45.00%	[1]
Discounted cash flow | Level 3 | U.S. government and agency
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.18%	[1]	2.57%	[1]
Discounted cash flow | Level 3 | State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.88%	[1]	2.94%	[1]
Discounted cash flow | Level 3 | Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.81%	[1]	3.01%	[1]
Discounted cash flow | Level 3 | Collateralized Loan Obligations
Fair Value, Option, Quantitative Disclosures [Line Items]
Default rate			2.50%	[1]
Recovery rate			65.00%	[1]
Prepayment rate			20.00%	[1]
Reinvestment rate			4.00%	[1]
Discounted cash flow | Level 3 | High Yield Bonds
Fair Value, Option, Quantitative Disclosures [Line Items]
Recovery rate			35.00%	[1]
Discounted cash flow | Level 3 | Investment Grade Bonds
Fair Value, Option, Quantitative Disclosures [Line Items]
Recovery rate			45.00%	[1]
Discounted cash flow | Level 3 | Other asset-backed
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.30%	[1]	4.97%	[1]
Default rate	2.53%	[1]	2.53%	[1]
Default rate term 2	0.37%	[1]	0.33%	[1]
Recovery rate	10.00%	[1]	10.00%	[1]
Discounted cash flow | Minimum | Level 3 | U.S. government and agency
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	1.07%	[1]	1.46%	[1]
Discounted cash flow | Minimum | Level 3 | State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.44%	[1]	1.94%	[1]
Discounted cash flow | Minimum | Level 3 | Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	1.06%	[1]	1.47%	[1]
Discounted cash flow | Minimum | Level 3 | Other asset-backed
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	2.10%	[1]	2.61%	[1]
Discounted cash flow | Maximum | Level 3 | U.S. government and agency
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	5.00%	[1]	3.29%	[1]
Discounted cash flow | Maximum | Level 3 | State and political subdivision
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	5.79%	[1]	3.53%	[1]
Discounted cash flow | Maximum | Level 3 | Corporate
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	6.75%	[1]	6.33%	[1]
Discounted cash flow | Maximum | Level 3 | Other asset-backed
Fair Value, Option, Quantitative Disclosures [Line Items]
Yield	3.41%	[1]	9.50%	[1]

[1] Excludes Level 3 assets which are valued based upon non-binding independent third-party valuations or third-party price information for which unobservable inputs are not reasonably available to us.