Consolidated Statements of Comprehensive Income (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Statement of Other Comprehensive Income [Abstract]
Net income	$ 148.2	$ 656.2	$ 800.1	$ 885.2	$ 464.2	$ 691.2	$ 643.6	$ 856.5	$ 2,489.7	$ 2,655.5	$ 2,646.7
Other comprehensive (loss) income, net of tax:
Change in net unrealized gains/losses on investments									(294.7)	189.9	20.6
Change in non-credit component of other-than-temporary impairment losses on investments									1.7	4.5	(0.7)
Change in net unrealized gains/losses on cash flow hedges									3.0	0.1	(10.0)
Change in net periodic pension and postretirement benefits									172.7	(10.9)	(119.8)
Foreign currency translation adjustments									1.4	0.6	0.2
Other comprehensive (loss) income									(115.9)	184.2	(109.7)
Total comprehensive income									$ 2,373.8	$ 2,839.7	$ 2,537.0