Investments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Schedule of Investments [Line Items]
Available-for-sale Securities	$ 20,666.0		$ 20,666.0		$ 19,254.9
Net unrealized gains on investments	660.6	476.6	660.6	476.6
Other-than-temporary impairment losses recognized in income	13.8	26.2	35.5	73.1
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	2,726.9		2,726.9
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	625.8		625.8
Sub-Prime Mortgage-Backed and Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	24.8		24.8
Unrealized Gain (Loss) on Securities	1.6
Alt-A Mortgage-Backed and Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	88.4		88.4
Unrealized Gain (Loss) on Securities	$ 6.5