Consolidated Statement of Stockholders' 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	Preferred Stock	Preferred Stock Preferred Stock	Preferred Stock Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principles, net of taxes	$ 7.5				$ 41.4	$ (33.9)
Beginning balance at Dec. 31, 2017	4,281.5	$ 0.0	$ 1.5	$ 3,197.9	5,697.3	234.0	$ (4,860.1)	$ 10.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock plan exercises	(8.8)			(8.8)
Stock plan compensation expense	21.7			21.7
Common stock dividends	(60.6)				(60.6)
Net income	173.0				173.0
Issuance of stock	975.5		0.1	975.4					$ 276.4	$ 2.9	$ 273.5
Preferred stock dividends	(4.8)				(4.8)
Change in equity of non-controlling interests	9.5							9.5
Other comprehensive loss	(338.0)					(338.0)
Ending balance at Jun. 30, 2018	5,332.9	2.9	1.6	4,459.7	5,846.3	(137.9)	(4,860.1)	20.4
Beginning balance at Mar. 31, 2018	4,501.0	2.9	1.5	3,478.8	5,815.0	51.7	(4,860.1)	11.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock plan exercises	(6.6)			(6.6)
Stock plan compensation expense	12.1			12.1
Common stock dividends	(30.9)				(30.9)
Net income	67.0				67.0
Issuance of stock	975.5		0.1	975.4
Preferred stock dividends	(4.8)				(4.8)
Change in equity of non-controlling interests	9.2							9.2
Other comprehensive loss	(189.6)					(189.6)
Ending balance at Jun. 30, 2018	5,332.9	2.9	1.6	4,459.7	5,846.3	(137.9)	(4,860.1)	20.4
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 exercises	(12.1)			(12.1)
Stock plan compensation expense	25.7			25.7
Common stock dividends	(75.7)				(75.7)
Acquisition of common stock	(100.7)						(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 loss	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 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 exercises	(1.0)			(1.0)
Stock plan compensation expense	14.2			14.2
Common stock dividends	(38.3)				(38.3)
Acquisition of common stock	(50.1)						(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							0.5
Other comprehensive loss	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