Investing Activities - Narrative (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013 counterparty	Mar. 31, 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%
Unrealized losses on below-investment-grade debt securities depressed of amortized cost, amount	$ 16.7		$ 21.6
Unrealized losses on below-investment-grade debt securities depressed of amortized cost for more than 12 month, amount	16.7		21.6
Unrealized losses on below-investment-grade debt securities depressed of amortized cost for more than 12 month, percentage	20.00%		20.00%
Total debt impairments recognized	1.0	1.0
Amount of impairments recognized as adjustment to other comprehensive loss	(0.7)	(0.3)
Limited partnerships and other investment OTTIs	5.7		6.7
Carrying value of investments in non-consolidated VIEs	8.6		2.1
Fixed maturities below-investment-grade assets	186.9		182.5
Derivative assets, net of liabilities	110.2
Number of counterparties	11
Debt securities, fair value	35.4
Maximum amount of loss due to credit risk	$ 145.6
Stockholders' Equity, Total [Member]
Concentration Risk [Line Items]
Concentration risk (percent)	10.00%