Equity - Stock Option Activity (Details) - Employee Stock Option - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Options
Beginning balance (in shares)	3,878,705	3,872,755	3,814,335
Granted (in shares)	730,913	751,615	882,335
Exercised (in shares)	(596,230)	(745,665)	(823,915)
Ending balance (in shares)	4,013,388	3,878,705	3,872,755
Weighted Average Exercise Price per Option
Weighted average beginning balance (in dollars per share)	$ 36.05	$ 31.93	$ 27.21
Granted (in dollars per share)	57.34	50.01	41.59
Exercised (in dollars per share)	27.97	28.73	20.40
Weighted average ending balance (in dollars per share)	$ 41.13	$ 36.05	$ 31.93