Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 472.8	$ 359.9	$ 1,160.7	$ 955.3
Other comprehensive income (loss), net of tax:
Net unrealized holding gains (losses) arising during the period	(17.7)	(100.2)	177.7	(91.0)
Reclassification of net (gains) losses on sales included in investment income	6.3	(2.6)	6.3	3.4
Income tax benefit (expense)	(65.3)	0.0	(65.9)	1.2
Investment securities, net	(89.3)	(97.6)	105.5	(93.2)
Net change in defined benefit plans arising during the period	0.0	0.0	3.1	(0.3)
Amortization of net actuarial (gains) losses included in compensation and benefits expense	0.8	0.6	2.4	2.0
Income tax benefit (expense)	(0.3)	(0.2)	(2.1)	(0.6)
Defined benefit plans, net	0.5	0.4	3.4	1.1
Net unrealized holding gains (losses) arising during the period	0.0	0.0	0.0	(4.7)
Ineffectiveness on cash flow hedges included in (gains) losses on derivative investments	0.0	0.0	0.0	1.8
Amortization of effective portion of net (gains) losses on cash flow hedges included in interest expense	(0.3)	(0.3)	(0.9)	(0.9)
Income tax benefit (expense)	0.1	0.1	0.3	1.5
Derivative investments, net	(0.2)	(0.2)	(0.6)	(2.3)
Foreign currency translation adjustments	(0.6)	(3.4)	(5.6)	(10.0)
Income tax benefit (expense)	(0.8)	1.1	1.0	3.6
Foreign currency translation, net	(1.4)	(2.3)	(4.6)	(6.4)
Other comprehensive income (loss), net of tax	(90.4)	(99.7)	103.7	(100.8)
Comprehensive Income	$ 382.4	$ 260.2	$ 1,264.4	$ 854.5