Shareholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jul. 31, 2018	Feb. 16, 2018
Class of Stock [Line Items]
Stock repurchase program, authorized				$ 1,000.0
Subsequent Event [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization			$ 654.5
Class A Common Stock [Member]
Class of Stock [Line Items]
Stock repurchased, shares	1.9	2.1
Stock repurchased, value	$ 306.9	$ 345.5
Weighted average price per share	$ 163.44	$ 163.23
Previous Stock Repurchase Program [Member]
Class of Stock [Line Items]
Stock repurchase program, remaining authorization				$ 150.0