Basis of Presentation and Summary of Significant Accounting Policies (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013	Dec. 31, 2012
Investments and Cash
Performance fees incurred	$ 1.5	$ 0.9
Performance fees incurred (reversed) related to investment-related derivative instruments	(0.7)	0.3
Cash and cash equivalents	747.4	352.8	468.4	283.4
The term of fixed income investments, as measured from the date of purchase, must be less than this term in order to be classified as cash and cash equivalents	three months		three months
Cash and cash equivalents held by investment advisors pending investment	365.6		120.9
Cash and cash equivalents held for operating expenses, including a provision for losses	30.6		28.0
Cash and cash equivalents held for other obligations and contingencies	14.9		4.9
Restricted cash	74.5		133.7
Collateral supporting investment securities sold short and derivative positions	60.3		121.1
Foreign Deposit Accounts held at Lloyd's	14.2		12.6
Repurchase and Reverse Repurchase Agreements
Amounts payable under repurchase agreements	402.7		0
Amounts receivable under reverse repurchase agreements	121.5		80.8
Collateralized Reinsurance Operations
Investment disclosures
Collateral earmarked for and held in trust	$ 336.3		$ 314.6