Investing Activities - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 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	$ 13.9		$ 13.9		$ 21.6
Unrealized losses on below-investment-grade debt securities depressed of amortized cost for more than 12 month, amount	13.9		13.9		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	0.6	1.2	1.6
Amount of impairments recognized as adjustment to other comprehensive loss	(0.2)	2.4	(0.9)	2.1
Limited partnerships and other investment OTTIs			7.2		6.7
Carrying value of investments in non-consolidated VIEs	10.1		10.1		2.1
Fixed maturities below-investment-grade assets	193.8		193.8		182.5
Derivative assets, net of liabilities	121.9		121.9
Debt securities, fair value	36.1		36.1
Maximum amount of loss due to credit risk			$ 158.0
Stockholders' Equity, Total [Member]
Concentration Risk [Line Items]
Concentration risk (percent)			10.00%