Stock compensation - RSU and PSU award activity (Details) - Restricted Stock Units (RSUs)	3 Months Ended
	Mar. 31, 2018 $ / shares shares
Shares Available For Grant
Shares Available for Grant Beginning Balance (in shares)	769,202
Granted (in shares)	(92,290)
Cancelled (in shares)	27,982
Shares Available for Grant Ending Balance (in shares)	704,894
Shares Subject to Outstanding Options, Shares
Outstanding at the beginning of the period (in shares)	1,178,660
Granted (in shares)	92,290
Cancelled (in shares)	(27,982)
Released (in shares)	(160,123)
Outstanding at the end of the period (in shares)	1,082,845
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share) | $ / shares	$ 98.88
Granted (in dollars per share) | $ / shares	93.62
Cancelled (in dollars per share) | $ / shares	104.64
Released (in dollars per share) | $ / shares	78.41
Outstanding at the end of the period (in dollars per share) | $ / shares	$ 101.31