Stockholders' Deficit - Option activity (Details) - Stock Options - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended
	Jul. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shares Available For Grant
Outstanding at the beginning of the period (in shares)			6,327,138
Options granted (in shares)			(2,562,773)
Options cancelled (in shares)			145,336
Outstanding at the end of the period (in shares)			3,909,701	6,327,138
Shares Subject to Outstanding Options, Shares
Outstanding at the beginning of the period (in shares)			11,504,572
Options granted (in shares)			2,562,773
Options exercised (in shares)			(2,715,456)
Options cancelled (in shares)			(145,336)
Outstanding at the end of the period (in shares)			11,206,553	11,504,572
Shares Subject to Outstanding Options, Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 48.40
Options granted (in dollars per share)			119.48
Options exercised (in dollars per share)			30.50
Options cancelled (in dollars per share)			97.30
Outstanding at the end of the period (in dollars per share)			$ 68.36	$ 48.40
Termination period	10 years	7 years
Vesting period	4 years	3 years
Stock option, other disclosures
Options exercisable and vested (in shares)			7,250,283	7,995,735	8,239,929
Intrinsic value of options exercised			$ 264.2	$ 137.0	$ 416.3
Intrinsic value of options outstanding			362.8
Intrinsic value of vested options			$ 362.0
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