Investments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Callable bonds without a make-whole provision of fixed maturity available-for-sale securities, percentage	8.30%
Callable bonds without a make-whole provision of fixed maturity available-for-sale securities, amount	$ 592.4
Securities deposited for the benefit of policyholders in accordance with state regulations	8.0		8.0		6.7
Interest income recognized on impaired commercial mortgage loans	1.0	0.7	3.1	2.8
Cash received by the company as interest on impaired commercial mortgage loans	1.0	0.7	3.0	2.1
Commercial mortgage loans that were at least 60 days past due	15.6		15.6		17.1
60 day delinquency proportion of commercial mortgage loan portfolio	0.30%		0.30%		0.34%
Total fixed maturity securities-available-for-sale	7,120.4		7,120.4		6,769.5
Investments And Commercial Banks In Euro Zone [Member]
Total fixed maturity securities-available-for-sale	$ 54.7		$ 54.7
Total fixed maturity securities-available-for-sale, Percentage	0.80%		0.80%