Stock compensation - RSU and PSU award activity (Details) - $ / shares	1 Months Ended			6 Months Ended
	Jul. 31, 2019	Jul. 31, 2018	Jun. 30, 2018	Jun. 30, 2020
Shares Subject to Outstanding Options, Shares
Outstanding at the beginning of the period (in shares)				2,602,376
Outstanding at the end of the period (in shares)				2,719,369
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)				$ 79.69
Outstanding at the end of the period (in dollars per share)				$ 80.33
Restricted Stock Units (RSUs)
Shares Subject to Outstanding Options, Shares
Granted (in shares)			190,000	415,017
Released (in shares)				(92,818)
Cancelled (in shares)				(62,956)
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Granted (in dollars per share)				$ 81.56
Released (in dollars per share)				85.93
Cancelled (in dollars per share)				$ 80.10
Performance Stock Units (PSUs)
Shares Subject to Outstanding Options, Shares
Granted (in shares)	86,975	77,243	446,500
Cancelled (in shares)				(142,250)
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Cancelled (in dollars per share)				$ 68.79