Investing Activities - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Concentration Risk [Line Items]
Unrealized losses on below-investment-grade debt securities depressed of amortized cost, percentage	20.00%		20.00%		20.00%
Unrealized losses on below-investment-grade debt securities depressed of amortized cost, amount	$ 6.9		$ 6.9		$ 21.6
Unrealized losses on below-investment-grade debt securities depressed of amortized cost for more than 12 month, amount	6.7		6.7		21.6
Unrealized losses on below-investment-grade debt securities depressed of amortized cost for more than 12 month, percentage	20.00%		20.00%		20.00%
Total debt impairments recognized	0.2	1.0	1.4	2.6
Amount of impairments recognized as adjustment to other comprehensive loss	0	(0.3)	(0.9)	1.8
Carrying value of investments in non-consolidated VIEs	7.9		7.9		2.1
Fixed maturities below-investment-grade assets	178.4		178.4		182.5
Derivative assets, net of liabilities	108.0		108.0
Debt securities, fair value	40.6		40.6
Maximum amount of loss due to credit risk	148.6
Stockholders' Equity, Total [Member]
Concentration Risk [Line Items]
Concentration risk (percent)	10.00%
Private Equity Investment
Concentration Risk [Line Items]
Equity method investments	7.4		7.4		6.7
Direct Equity Investment
Concentration Risk [Line Items]
Equity method investments	$ 2.9		$ 2.9