Derivative Financial Instruments (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012 Y	Dec. 31, 2011 Y
Credit derivatives sold disclosures
Reduction in total maximum future payments due to purchased credit protection	$ 10.0	$ 20.0
Net asset (liability) fair value of purchased credit derivative transactions	0.4	0
Notional amount	910.6	923.3
Fair value	(131.1)	(165.5)
Maximum future payments	910.6	923.3
Weighted average expected life (in years)	3.1	3.1
Single name credit default swaps
Credit derivatives sold disclosures
Notional amount	715.6	710.9
Fair value	(20.9)	(24.5)
Maximum future payments	715.6	710.9
Weighted average expected life (in years)	2.6	2.6
Single name credit default swaps | Corporate debt securities | AA
Credit derivatives sold disclosures
Notional amount	80.0	85.0
Fair value	(0.4)	(1.0)
Maximum future payments	80.0	85.0
Weighted average expected life (in years)	3.6	4.0
Single name credit default swaps | Corporate debt securities | A
Credit derivatives sold disclosures
Notional amount	498.0	483.0
Fair value	1.0	(1.4)
Maximum future payments	498.0	483.0
Weighted average expected life (in years)	2.3	2.5
Single name credit default swaps | Corporate debt securities | BBB
Credit derivatives sold disclosures
Notional amount	115.0	110.0
Fair value	(0.2)	(0.3)
Maximum future payments	115.0	110.0
Weighted average expected life (in years)	2.4	1.7
Single name credit default swaps | Corporate debt securities | CCC
Credit derivatives sold disclosures
Notional amount		10.0
Fair value		(0.1)
Maximum future payments		10.0
Weighted average expected life (in years)		0.2
Single name credit default swaps | Structured finance | C
Credit derivatives sold disclosures
Notional amount	10.0	10.0
Fair value	(8.9)	(8.9)
Maximum future payments	10.0	10.0
Weighted average expected life (in years)	9.9	10.1
Single name credit default swaps | Structured finance | Near default
Credit derivatives sold disclosures
Notional amount	12.6	12.9
Fair value	(12.4)	(12.8)
Maximum future payments	12.6	12.9
Weighted average expected life (in years)	0.9	1.2
Basket and index credit default swaps
Credit derivatives sold disclosures
Notional amount	195.0	212.4
Fair value	(110.2)	(141.0)
Maximum future payments	195.0	212.4
Weighted average expected life (in years)	4.9	4.8
Basket and index credit default swaps | Corporate debt securities | CCC
Credit derivatives sold disclosures
Notional amount		132.4
Fair value		(104.7)
Maximum future payments		132.4
Weighted average expected life (in years)		5.2
Basket and index credit default swaps | Corporate debt securities | CC
Credit derivatives sold disclosures
Notional amount		15.0
Fair value		(14.8)
Maximum future payments		15.0
Weighted average expected life (in years)		1.0
Basket and index credit default swaps | Corporate debt securities | Near default
Credit derivatives sold disclosures
Notional amount	140.0
Fair value	(94.0)
Maximum future payments	140.0
Weighted average expected life (in years)	4.7
Basket and index credit default swaps | Government/municipalities | AA
Credit derivatives sold disclosures
Notional amount	30.0
Fair value	(8.7)
Maximum future payments	30.0
Weighted average expected life (in years)	5.5
Basket and index credit default swaps | Government/municipalities | A
Credit derivatives sold disclosures
Notional amount		40.0
Fair value		(10.5)
Maximum future payments		40.0
Weighted average expected life (in years)		4.4
Basket and index credit default swaps | Structured finance | BBB
Credit derivatives sold disclosures
Notional amount	25.0	25.0
Fair value	(7.5)	(11.0)
Maximum future payments	$ 25.0	$ 25.0
Weighted average expected life (in years)	5.3	5.5