Derivative Financial Instruments (Details 3) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Credit derivatives sold disclosures
Reduction in total maximum future payments due to purchased credit protection	$ 44.9	$ 15.0
Net asset (liability) fair value of purchased credit derivative transactions	(0.5)	0.2
Notional amount	818.9	1,018.5
Fair value	(19.2)	(88.5)
Maximum future payments	818.9	1,018.5
Weighted average expected life	3 years 10 months 24 days	2 years 10 months 24 days
Single name credit default swaps
Credit derivatives sold disclosures
Notional amount	653.5	853.1
Fair value	5.1	(10.4)
Maximum future payments	653.5	853.1
Weighted average expected life	4 years	2 years 7 months 6 days
Single name credit default swaps | Corporate debt securities | AAA
Credit derivatives sold disclosures
Notional amount	10.0
Fair value	0.3
Maximum future payments	10.0
Weighted average expected life	4 years 8 months 12 days
Single name credit default swaps | Corporate debt securities | AA
Credit derivatives sold disclosures
Notional amount	84.0	70.0
Fair value	1.8	(0.2)
Maximum future payments	84.0	70.0
Weighted average expected life	4 years	2 years 6 months
Single name credit default swaps | Corporate debt securities | A
Credit derivatives sold disclosures
Notional amount	294.5	572.0
Fair value	4.2	2.4
Maximum future payments	294.5	572.0
Weighted average expected life	4 years	2 years 4 months 24 days
Single name credit default swaps | Corporate debt securities | BBB
Credit derivatives sold disclosures
Notional amount	265.0	200.0
Fair value	(1.2)	(1.6)
Maximum future payments	265.0	200.0
Weighted average expected life	3 years 10 months 24 days	3 years
Single name credit default swaps | Structured finance | Near default
Credit derivatives sold disclosures
Notional amount		11.1
Fair value		(11.0)
Maximum future payments		11.1
Weighted average expected life		8 years 6 months
Basket and index credit default swaps
Credit derivatives sold disclosures
Notional amount	165.4	165.4
Fair value	(24.3)	(78.1)
Maximum future payments	165.4	165.4
Weighted average expected life	3 years 4 months 24 days	4 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	(19.9)	(65.2)
Maximum future payments	110.4	110.4
Weighted average expected life	3 years 2 months 12 days	4 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	(3.5)	(7.3)
Maximum future payments	30.0	30.0
Weighted average expected life	3 years 8 months 12 days	4 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	(0.9)	(5.6)
Maximum future payments	$ 25.0	$ 25.0
Weighted average expected life	3 years 6 months	4 years 6 months