Derivative Financial Instruments (Details 3) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011 Y	Dec. 31, 2010 Y
Credit derivatives sold disclosures
Reduction in total maximum future payments due to purchased credit protection	$ 20.0	$ 10.0
Net asset (liability) fair value of purchased credit derivative transactions	0	(0.8)
Notional amount	923.3	1,107.9
Fair value	(165.5)	(162.7)
Maximum future payments	923.3	1,107.9
Weighted average expected life (in years)	3.1	3.5
Single name credit default swaps
Credit derivatives sold disclosures
Notional amount	710.9	896.9
Fair value	(24.5)	(37.7)
Maximum future payments	710.9	896.9
Weighted average expected life (in years)	2.6	3.0
Single name credit default swaps | Corporate debt securities | AA
Credit derivatives sold disclosures
Notional amount	85.0	135.0
Fair value	(1.0)	(0.5)
Maximum future payments	85.0	135.0
Weighted average expected life (in years)	4.0	3.9
Single name credit default swaps | Corporate debt securities | A
Credit derivatives sold disclosures
Notional amount	483.0	564.0
Fair value	(1.4)	0.9
Maximum future payments	483.0	564.0
Weighted average expected life (in years)	2.5	2.9
Single name credit default swaps | Corporate debt securities | BBB
Credit derivatives sold disclosures
Notional amount	110.0	150.0
Fair value	(0.3)	0.3
Maximum future payments	110.0	150.0
Weighted average expected life (in years)	1.7	1.1
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 | B
Credit derivatives sold disclosures
Notional amount		25.9
Fair value		(20.0)
Maximum future payments		25.9
Weighted average expected life (in years)		5.9
Single name credit default swaps | Structured finance | CCC
Credit derivatives sold disclosures
Notional amount		22.0
Fair value		(18.4)
Maximum future payments		22.0
Weighted average expected life (in years)		9.4
Single name credit default swaps | Structured finance | C
Credit derivatives sold disclosures
Notional amount	10.0
Fair value	(8.9)
Maximum future payments	10.0
Weighted average expected life (in years)	10.1
Single name credit default swaps | Structured finance | Near default
Credit derivatives sold disclosures
Notional amount	12.9
Fair value	(12.8)
Maximum future payments	12.9
Weighted average expected life (in years)	1.2
Basket and index credit default swaps
Credit derivatives sold disclosures
Notional amount	212.4	211.0
Fair value	(141.0)	(125.0)
Maximum future payments	212.4	211.0
Weighted average expected life (in years)	4.8	5.6
Basket and index credit default swaps | Corporate debt securities | A
Credit derivatives sold disclosures
Notional amount		6.0
Maximum future payments		6.0
Weighted average expected life (in years)		1.0
Basket and index credit default swaps | Corporate debt securities | CCC
Credit derivatives sold disclosures
Notional amount	132.4	125.0
Fair value	(104.7)	(103.0)
Maximum future payments	132.4	125.0
Weighted average expected life (in years)	5.2	6.2
Basket and index credit default swaps | Corporate debt securities | CC
Credit derivatives sold disclosures
Notional amount	15.0	15.0
Fair value	(14.8)	(8.5)
Maximum future payments	15.0	15.0
Weighted average expected life (in years)	1.0	2.0
Basket and index credit default swaps | Government/municipalities | A
Credit derivatives sold disclosures
Notional amount	40.0	40.0
Fair value	(10.5)	(11.2)
Maximum future payments	40.0	40.0
Weighted average expected life (in years)	4.4	5.4
Basket and index credit default swaps | Structured finance | AA
Credit derivatives sold disclosures
Notional amount		20.0
Fair value		(2.0)
Maximum future payments		20.0
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	5.0
Fair value	(11.0)	(0.3)
Maximum future payments	$ 25.0	$ 5.0
Weighted average expected life (in years)	5.5	14.9