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, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Credit Losses Recognized in Earnings on Debt Securities for which a Portion of the OTTI Loss was Recognized in OCI:
Balance, beginning of period	$ (17.9)		$ (18.4)		$ (18.6)		$ (17.8)
Add: Credit losses on securities not previously impaired	0	[1]	0	[1]	0	[1]	0	[1]
Add: Credit losses on securities previously impaired	0	[1]	(0.1)	[1]	0	[1]	(0.7)	[1]
Less: Credit losses on securities impaired due to intent to sell	0		0		0		0
Less: Credit losses on securities sold	0.4		0		1.1		0
Less: Increases in cash flows expected on previously impaired securities	0		0		0		0
Balance, end of period	$ (17.5)		$ (18.5)		$ (17.5)		$ (18.5)

[1] Additional credit losses on securities for which a portion of the OTTI loss was recognized in AOCI are included within net OTTI losses recognized in earnings on the statements of income and comprehensive income.