Derivative Financial Instruments (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Credit derivatives sold disclosures
Reduction in total maximum future payments due to purchased credit protection	$ 44.9	$ 44.9
Net asset (liability) fair value of purchased credit derivative transactions	(0.4)	(0.5)
Notional amount	828.9	818.9
Fair value	(15.3)	(19.2)
Maximum future payments	828.9	818.9
Weighted average expected life	3 years 7 months 6 days	3 years 10 months 24 days
Single name credit default swaps
Credit derivatives sold disclosures
Notional amount	663.5	653.5
Fair value	5.8	5.1
Maximum future payments	663.5	653.5
Weighted average expected life	3 years 9 months 18 days	4 years
Single name credit default swaps | Corporate debt securities | AAA
Credit derivatives sold disclosures
Notional amount	20.0	10.0
Fair value	0.6	0.3
Maximum future payments	20.0	10.0
Weighted average expected life	4 years 1 month 6 days	4 years 8 months 12 days
Single name credit default swaps | Corporate debt securities | AA
Credit derivatives sold disclosures
Notional amount	79.0	84.0
Fair value	1.5	1.8
Maximum future payments	79.0	84.0
Weighted average expected life	3 years 9 months 18 days	4 years
Single name credit default swaps | Corporate debt securities | A
Credit derivatives sold disclosures
Notional amount	289.5	294.5
Fair value	3.9	4.2
Maximum future payments	289.5	294.5
Weighted average expected life	3 years 9 months 18 days	4 years
Single name credit default swaps | Corporate debt securities | BBB
Credit derivatives sold disclosures
Notional amount	275.0	265.0
Fair value	(0.2)	(1.2)
Maximum future payments	275.0	265.0
Weighted average expected life	3 years 8 months 12 days	3 years 10 months 24 days
Basket and index credit default swaps
Credit derivatives sold disclosures
Notional amount	165.4	165.4
Fair value	(21.1)	(24.3)
Maximum future payments	165.4	165.4
Weighted average expected life	3 years 1 month 6 days	3 years 4 months 24 days
Basket and index credit default swaps | Corporate debt securities | Near default
Credit derivatives sold disclosures
Notional amount	110.4	110.4
Fair value	(17.8)	(19.9)
Maximum future payments	110.4	110.4
Weighted average expected life	3 years	3 years 2 months 12 days
Notional amount of derivative whose credit risk is borne by third party investors	88.0	88.0
Basket and index credit default swaps | Government/municipalities | AA
Credit derivatives sold disclosures
Notional amount	30.0	30.0
Fair value	(2.9)	(3.5)
Maximum future payments	30.0	30.0
Weighted average expected life	3 years 6 months	3 years 8 months 12 days
Basket and index credit default swaps | Structured finance | BBB
Credit derivatives sold disclosures
Notional amount	25.0	25.0
Fair value	(0.4)	(0.9)
Maximum future payments	$ 25.0	$ 25.0
Weighted average expected life	3 years 3 months 18 days	3 years 6 months