Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 746.9	$ 617.8	$ 2,612.1	$ 2,101.4
Other comprehensive income (loss), net of tax:
Change in net unrealized gains/losses on investments	8.6	5.3	189.7	387.4
Change in non-credit component of other-than-temporary impairment losses on investments	0.0	9.3	4.5	2.2
Change in net unrealized losses on cash flow hedges	(4.9)	(17.2)	(67.7)	(472.7)
Change in net periodic pension and postretirement costs	4.7	3.2	12.8	10.8
Foreign currency translation adjustments	0.5	0.4	2.6	1.0
Other comprehensive income (loss)	8.9	1.0	141.9	(71.3)
Total comprehensive income	$ 755.8	$ 618.8	$ 2,754.0	$ 2,030.1