Consolidated Statement of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Treasury Stock
Beginning balance at Dec. 31, 2015					$ 118.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 534.1
Other comprehensive income	209.5
Ending balance at Sep. 30, 2016					328.0
Beginning balance at Jun. 30, 2016					327.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	144.4
Other comprehensive income	0.9
Ending balance at Sep. 30, 2016					328.0
Beginning balance at Dec. 31, 2016	4,098.1	$ 1.5	$ 3,175.9	$ 5,296.7	94.6	$ (4,470.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock plan exercises	(12.1)		(12.1)
Stock plan compensation	23.0		23.0
Dividends	(88.8)			(88.8)
Acquisition of common stock	(250.5)					(250.5)
Net income	206.7			206.7
Other comprehensive income	161.8				161.8
Ending balance at Sep. 30, 2017	4,138.2	1.5	3,186.8	5,414.6	256.4	(4,721.1)
Beginning balance at Jun. 30, 2017					236.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(57.3)
Other comprehensive income	19.5
Ending balance at Sep. 30, 2017	$ 4,138.2	$ 1.5	$ 3,186.8	$ 5,414.6	$ 256.4	$ (4,721.1)