Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 1,009.9	$ 703.0
Other comprehensive income (loss), net of tax:
Change in net unrealized gains/losses on investments	80.2	172.3
Change in non-credit component of other-than-temporary impairment losses on investments	3.6	(1.7)
Change in net unrealized gains/losses on cash flow hedges	17.0	(265.5)
Change in net periodic pension and postretirement costs	3.9	3.8
Foreign currency translation adjustments	1.4	1.3
Other comprehensive income (loss)	106.1	(89.8)
Total comprehensive income	$ 1,116.0	$ 613.2