Stock compensation - Option activity (Details) - Stock Options - $ / shares	1 Months Ended	3 Months Ended	6 Months Ended
	Jul. 31, 2016	Mar. 31, 2018	Jun. 30, 2016
Shares Available For Grant
Outstanding at the beginning of the period (in shares)		3,909,701
Options granted (in shares)		(1,308,438)
Options cancelled (in shares)		139,853
Outstanding at the end of the period (in shares)		2,741,116
Shares Subject to Outstanding Options, Shares
Outstanding at the beginning of the period (in shares)		11,206,553
Options granted (in shares)		1,308,438
Options exercised (in shares)		(486,605)
Options cancelled (in shares)		(139,853)
Outstanding at the end of the period (in shares)		11,888,533
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)		$ 68.36
Options granted (in dollars per share)		94.59
Options exercised (in dollars per share)		22.17
Options cancelled (in dollars per share)		102.47
Outstanding at the end of the period (in dollars per share)		$ 72.74
Termination period	10 years		7 years
Vesting period	4 years		3 years
Vesting after one year
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Vesting period	1 year		1 year
Vesting Percentage	25.00%		33.00%
Remainder vesting
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Vesting period	36 months		24 months