Stock Compensation - Stock Option Activity (Details) shares in Millions	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Shares Subject to Option
Outstanding at December 31, 2017 (in shares) | shares	18.6
Exercised (in shares) | shares	1.1
Outstanding and Exercisable at December 31, 2018 (in shares) | shares	17.5
Weighted Average Exercise Price
Outstanding at December 31, 2017 (in dollars per share) | $ / shares	$ 41.68
Exercised (in dollars per share) | $ / shares	37.82
Outstanding and Exercisable at December 31, 2018 (in dollars per share) | $ / shares	$ 41.92