Derivative Financial Instruments (Details 3) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Credit derivatives sold disclosures
Reduction in total maximum future payments due to purchased credit protection	$ 10.0	$ 10.0
Net asset (liability) fair value of purchased credit derivative transactions	0	(0.1)
Credit default swaps
Credit derivatives sold disclosures
Notional amount	904.8	945.8
Fair value	(14.1)	(12.2)
Maximum future payments	904.8	945.8
Weighted average expected life	3 years 2 months 12 days	3 years 3 months 18 days
Single name credit default swaps
Credit derivatives sold disclosures
Notional amount	758.5	798.5
Fair value	8.7	8.6
Maximum future payments	758.5	798.5
Weighted average expected life	3 years 6 months	3 years 6 months
Single name credit default swaps | Corporate debt securities | AAA
Credit derivatives sold disclosures
Notional amount	30.0	30.0
Fair value	1.0	1.0
Maximum future payments	30.0	30.0
Weighted average expected life	4 years	4 years 2 months 12 days
Single name credit default swaps | Corporate debt securities | AA
Credit derivatives sold disclosures
Notional amount	74.0	79.0
Fair value	1.4	1.6
Maximum future payments	74.0	79.0
Weighted average expected life	3 years	3 years 3 months 18 days
Single name credit default swaps | Corporate debt securities | A
Credit derivatives sold disclosures
Notional amount	229.5	254.5
Fair value	3.5	3.3
Maximum future payments	229.5	254.5
Weighted average expected life	2 years 10 months 24 days	2 years 9 months 18 days
Single name credit default swaps | Corporate debt securities | BBB
Credit derivatives sold disclosures
Notional amount	335.0	345.0
Fair value	2.1	1.2
Maximum future payments	335.0	345.0
Weighted average expected life	3 years 7 months 6 days	3 years 7 months 6 days
Single name credit default swaps | Corporate debt securities | BB
Credit derivatives sold disclosures
Notional amount	10.0	10.0
Fair value	0.5	0.9
Maximum future payments	10.0	10.0
Weighted average expected life	4 years 8 months 12 days	5 years
Single name credit default swaps | Government/municipalities | AA
Credit derivatives sold disclosures
Notional amount	30.0	30.0
Fair value	0.6	0.6
Maximum future payments	30.0	30.0
Weighted average expected life	4 years 1 month 6 days	4 years 3 months 18 days
Single name credit default swaps | Sovereign | AA
Credit derivatives sold disclosures
Notional amount	10.0	10.0
Fair value	0.1	0.1
Maximum future payments	10.0	10.0
Weighted average expected life	4 years 6 months	4 years 8 months 12 days
Single name credit default swaps | Sovereign | BBB
Credit derivatives sold disclosures
Notional amount	40.0	40.0
Fair value	(0.5)	(0.1)
Maximum future payments	40.0	40.0
Weighted average expected life	4 years 6 months	4 years 8 months 12 days
Basket and index credit default swaps
Credit derivatives sold disclosures
Notional amount	146.3	147.3
Fair value	(22.8)	(20.8)
Maximum future payments	146.3	147.3
Weighted average expected life	2 years	2 years 6 months
Basket and index credit default swaps | Corporate debt securities | Near default
Credit derivatives sold disclosures
Notional amount	100.4	100.4
Fair value	(21.3)	(19.1)
Maximum future payments	100.4	100.4
Weighted average expected life	2 years	2 years 2 months 12 days
Notional amount of derivative whose credit risk is borne by third party investors	78.0	78.0
Basket and index credit default swaps | Government/municipalities | AA
Credit derivatives sold disclosures
Notional amount	30.0	30.0
Fair value	(1.6)	(1.8)
Maximum future payments	30.0	30.0
Weighted average expected life	2 years 6 months	2 years 8 months 12 days
Basket and index credit default swaps | Structured finance | BBB
Credit derivatives sold disclosures
Notional amount	15.9	16.9
Fair value	0.1	0.1
Maximum future payments	$ 15.9	$ 16.9
Weighted average expected life	1 year 4 months 24 days	3 years 6 months