Investments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2012 security	Sep. 30, 2011	Sep. 30, 2012 security	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Minimum [Member]	Jun. 30, 2012 Maximum [Member]	Sep. 30, 2012 Pooled Bank Trust Preferred Securities [Member]	Dec. 31, 2011 Pooled Bank Trust Preferred Securities [Member]	Dec. 31, 2011 Community Bank And Bank Holding Equity [Member]
Schedule of Investments [Line Items]
Equity ownership positions in the community bank holding companies						2.00%	5.00%
Credit-related net investment impairment losses			$ 0.6		$ 1.3			$ 0.6	$ 0.4	$ 0.9
Pooled trust preferred securities, book value								5.6
Remaining book value									6.3	3.9
Pooled trust preferred securities, estimated fair value	3.7		3.7
Number of pooled trust preferred securities	5		5
Regional community bank stocks, percent of average trading volumes							0.10%
Annual prepayment rate			1.00%
Annual prepayments maturity	100.00%		100.00%
Additional basis points	150.00%		150.00%
Gross gains from investment security transactions	0.7	0.6	1.5	3.8
Carrying value of securities pledged	$ 218		$ 218		$ 204