Stock compensation - RSU and PSU award activity (Details) - $ / shares	1 Months Ended		9 Months Ended
	Jul. 31, 2018	Jun. 30, 2018	Sep. 30, 2018
Restricted Stock Units (RSUs)
Shares Available For Grant
Shares Available for Grant Beginning Balance (in shares)			769,202
Additional authorization (in shares)			1,000,000
Granted (in shares)			(867,197)
Cancelled (in shares)			87,983
Shares Available for Grant Ending Balance (in shares)			472,995
Shares Subject to Outstanding Options, Shares
Outstanding at the beginning of the period (in shares)			1,178,660
Granted (in shares)		190,000,000	867,197
Cancelled (in shares)			(87,983)
Released (in shares)			(337,357)
Outstanding at the end of the period (in shares)			2,137,510
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 98.88
Granted (in dollars per share)			70.48
Cancelled (in dollars per share)			97.19
Released (in dollars per share)			92.88
Outstanding at the end of the period (in dollars per share)			$ 80.47
Performance Stock Units (PSUs)
Shares Available For Grant
Granted (in shares)			(523,743)
Cancelled (in shares)			6,750
Shares Subject to Outstanding Options, Shares
Granted (in shares)	77,243	446,500,000	523,743
Cancelled (in shares)			(6,750)
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Granted (in dollars per share)			$ 66.18
Cancelled (in dollars per share)			$ 65.76