Investments (Narrative) (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	Dec. 31, 2013
Schedule of Investments [Line Items]
Available-for-sale Securities	$ 20,755.2		$ 20,755.2		$ 19,254.9
Net unrealized gains on investments	793.9	460.3	793.9	460.3
Other-than-temporary impairment losses recognized in income	10.9	9.0	21.7	46.9
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	2,751.6		2,751.6
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	536.7		536.7
Sub-Prime Mortgage-Backed and Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	28.4		28.4
Unrealized Gain (Loss) on Securities	1.3
Alt-A Mortgage-Backed and Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	94.2		94.2
Unrealized Gain (Loss) on Securities	$ 6.6