Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012 NumberOfDerivativeContracts	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
S & P 500 index options purchased per month	1
Notional amount of index option contract	$ 313.2		$ 313.2		$ 303.1
Derivative Option premiums paid	2.1	2.2	6.9	6.9
Amount received for derivative options exercised	3.6	3.8	7.6	12.8
Maximum credit exposure required increase in S&P 500 index, percentage	8.10%
Impact On Interest Credited Changes In Actuarial Assumptions	$ 1.1	$ 0.2	$ 2.2	$ 1.7
S and P Five Hundred Index Options [Member]
Expiry date of S & P 500 index options purchased, years	1 year 0 months 0 days