Fair Value Measurements (Details 6) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014 item	Sep. 30, 2013 item	Sep. 30, 2014 item	Sep. 30, 2013 item	Dec. 31, 2013
Commercial Mortgage Loans of Consolidated VIEs
Fair Value Option, Quantitative Disclosures
Fair value of assets for which fair value option was elected	$ 41.8		$ 41.8		$ 68.1
Contractual principal amounts of assets for which the fair value option was elected	38.5		38.5		64.0
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected	(0.2)	(0.5)	(0.8)	1.1
Credit risk portion of pre-tax gain (loss) due to change in fair value of assets for which the fair value option was elected	0	0	0	0
Number of loans which are more than 90 days past due or in nonaccrual status	0	0	0	0
Interest income	3.2	1.4	5.4	4.3
Obligations of Consolidated VIEs
Fair Value Option, Quantitative Disclosures
Fair value of liabilities for which the fair value option was elected	65.8		65.8		104.9
Aggregate unpaid principal amounts of obligations for which the fair value option was elected	138.9		138.9		174.4
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected	10.4	(7.2)	4.7	(26.3)
Credit risk portion of pre-tax gain (loss) due to change in fair value of liabilities for which the fair value option was elected	8.9	(7.6)	2.9	(27.5)
Interest expense	2.7	0.9	3.8	2.7
Equity Method Investment
Fair Value Option, Quantitative Disclosures
Fair value of assets for which fair value option was elected	82.4		82.4		74.8
Pre-tax gain (loss) due to change in fair value of assets and liabilities for which the fair value option was elected	$ 4.0	$ 2.3	$ 7.6	$ 6.1