Derivative Financial Instruments (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Derivative Financial Instruments
Future potential payments in the event of default of an investment in fixed maturity securities that contain embedded credit derivatives	$ 0
Hybrid instruments disclosures
Amortized cost	114.2	111.4
Carrying value	112.2	109.2
Weighted average expected life	3 years 7 months 6 days	3 years 9 months 18 days
Corporate debt securities
Hybrid instruments disclosures
Amortized cost	46.2	45.5
Carrying value	46.2	45.4
Weighted average expected life	1 year 10 months 24 days	2 years 1 month 6 days
Corporate debt securities | BBB
Hybrid instruments disclosures
Amortized cost	21.2	20.5
Carrying value	21.2	20.5
Weighted average expected life	3 years 9 months 18 days	4 years
Corporate debt securities | B
Hybrid instruments disclosures
Amortized cost	25.0	25.0
Carrying value	25.0	24.9
Weighted average expected life	3 months 18 days	6 months
Structured finance
Hybrid instruments disclosures
Amortized cost	68.0	65.9
Carrying value	66.0	63.8
Weighted average expected life	4 years 10 months 24 days	4 years 10 months 24 days
Structured finance | A
Hybrid instruments disclosures
Amortized cost	5.6
Carrying value	5.6
Weighted average expected life	16 years 10 months 24 days
Structured finance | AA
Hybrid instruments disclosures
Amortized cost		4.6
Carrying value		4.6
Weighted average expected life		17 years
Structured finance | BB
Hybrid instruments disclosures
Amortized cost	35.6	39.6
Carrying value	33.6	37.5
Weighted average expected life	2 years 6 months	2 years 10 months 24 days
Structured finance | B
Hybrid instruments disclosures
Amortized cost	4.1	4.0
Carrying value	4.1	4.0
Weighted average expected life	4 years 2 months 12 days	4 years 4 months 24 days
Structured finance | CCC
Hybrid instruments disclosures
Amortized cost	22.7	17.7
Carrying value	$ 22.7	$ 17.7
Weighted average expected life	5 years 8 months 12 days	6 years 4 months 24 days