Investments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Schedule of Investments [Line Items]
Available-for-sale Securities	$ 20,500.9		$ 19,254.9
Net unrealized gains on investments	649.3	838.0
Other-than-temporary impairment losses recognized in income	10.8	37.9
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	2,700.8
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	501.5
Sub-Prime Mortgage-Backed and Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities	30.0
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	98.8
Unrealized Gain (Loss) on Securities	$ 7.1