Investing Activities - Other Than Temporary Impairment, Credit Losses Recognized in Earnings (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Credit Losses Recognized in Earnings on Debt Securities for which a Portion of the OTTI Loss was Recognized in OCI:
Balance, beginning of period	$ (18.4)	$ (22.1)	$ (17.8)	$ (21.3)
Add: Credit losses on securities not previously impaired	0	0	0	(0.3)
Add: Credit losses on securities previously impaired	(0.1)	(0.5)	(0.7)	(1.0)
Less: Credit losses on securities impaired due to intent to sell	0	0	0	0
Less: Credit losses on securities sold	0	2.6	0	2.6
Less: Increases in cash flows expected on previously impaired securities	0	0	0	0
Balance, end of period	$ (18.5)	$ (20.0)	$ (18.5)	$ (20.0)