Consolidated Statements of Changes in Equity (unaudited) - USD ($) $ in Millions		Total	Preferred Stock	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Non-controlling Interests	Cumulative Effect, Period of Adoption, Adjustment	[1]	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings	[1]
Beginning balance at Dec. 31, 2018		$ 5,133.9	$ 2.9	$ 1.6	$ 4,495.6	$ 5,759.7	$ (155.4)	$ (4,992.4)	$ 21.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock plan issuances		3.5			3.5
Stock plan compensation expense		25.7			25.7
Common stock dividends		(75.7)				(75.7)
Acquisition of common stock		116.3			15.6			100.7
Net income		311.2				309.8			1.4
Preferred stock dividends		(9.3)				(9.3)
Change in equity of non-controlling interests		3.2				(3.6)			6.8
Other comprehensive income		484.1					484.1
Ending balance at Jun. 30, 2019		5,760.3	2.9	1.6	4,509.2	5,980.9	328.7	(5,093.1)	30.1
Beginning balance at Dec. 31, 2018		$ 5,133.9	2.9	1.6	4,495.6	5,759.7	(155.4)	(4,992.4)	21.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]	[1]	us-gaap:AccountingStandardsUpdate201613Member
Ending balance at Dec. 31, 2019		$ 5,682.1	2.9	1.6	4,537.7	5,966.4	411.5	(5,267.3)	29.3	$ (20.4)		$ (20.4)
Beginning balance at Mar. 31, 2019		5,471.7	2.9	1.6	4,496.0	5,879.7	103.4	(5,043.0)	31.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock plan issuances		0.0			0.0
Stock plan compensation expense		14.2			14.2
Common stock dividends		(38.3)				(38.3)
Acquisition of common stock		51.1			1.0			50.1
Net income		142.6				144.1			(1.5)
Preferred stock dividends		(4.6)				(4.6)
Change in equity of non-controlling interests		0.5				0.0			0.5
Other comprehensive income		225.3					225.3
Ending balance at Jun. 30, 2019		5,760.3	2.9	1.6	4,509.2	5,980.9	328.7	(5,093.1)	30.1
Beginning balance at Dec. 31, 2019		$ 5,682.1	2.9	1.6	4,537.7	5,966.4	411.5	(5,267.3)	29.3	$ (20.4)		$ (20.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member
Stock plan issuances		$ 3.9			3.9
Stock plan compensation expense		26.4			26.4
Common stock dividends		(77.6)				(77.6)
Acquisition of common stock		100.7			17.6			83.1
Net income		334.2				332.8			1.4
Preferred stock dividends		(9.3)				(9.3)
Change in equity of non-controlling interests		(11.5)				2.5			(14.0)
Other comprehensive income		157.4					157.4
Ending balance at Jun. 30, 2020		5,984.5	2.9	1.6	4,550.4	6,194.4	568.9	(5,350.4)	16.7
Beginning balance at Mar. 31, 2020		5,386.2	2.9	1.6	4,540.6	6,061.1	88.4	(5,324.6)	16.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock plan issuances		0.0			0.0
Stock plan compensation expense		14.9			14.9
Common stock dividends		(39.6)				(39.6)
Acquisition of common stock		30.9			5.1			25.8
Net income		178.4				178.1			0.3
Preferred stock dividends		(4.6)				(4.6)
Change in equity of non-controlling interests		(0.4)				(0.6)			0.2
Other comprehensive income		480.5					480.5
Ending balance at Jun. 30, 2020		$ 5,984.5	$ 2.9	$ 1.6	$ 4,550.4	$ 6,194.4	$ 568.9	$ (5,350.4)	$ 16.7

[1] Amount relates to the adoption of a new accounting standard for accounting for expected credit losses for assets held at amortized cost, which established allowances for such expected credit losses as of January 1, 2020. Refer to Notes 3 and 6 for additional information.