Shareholders' Equity - Schedule of Stock Repurchases (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended						4 Months Ended	9 Months Ended		90 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Oct. 25, 2019	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
Class of Stock [Line Items]
Total repurchase cost	$ 350.0	$ 1,056.0	$ 101.0	$ 301.0	$ 402.0	$ 553.0
2012 - Share Repurchase Program
Class of Stock [Line Items]
Shares purchased (in shares)	1,800			2,100				8,200	8,800	126,500
Average price per share of stock repurchased (in dollars per share)	$ 190.83			$ 145.71				$ 183.26	$ 142.15
Total repurchase cost	$ 350.0			$ 300.0				$ 1,500.0	$ 1,250.0	$ 12,500.0
Additional associated costs	0.0			1.0				7.0	6.0
Total costs recorded to retained earnings	$ 350.0			$ 301.0				$ 1,507.0	$ 1,256.0
Subsequent Event | 2012 - Share Repurchase Program
Class of Stock [Line Items]
Shares purchased (in shares)							25
Average price per share of stock repurchased (in dollars per share)							$ 193.91
Total repurchase cost							$ 4.8