Common Stock, Preferred Stock and Equity Plans - Performance-based RSUs (PSUs) (Details) - Performance-based RSUs - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Restricted stock and Restricted Stock Units
Beginning of the period (in shares)	484
Granted (in shares)	494
Vested (in shares)	(154)
Cancelled (in shares)	(65)
End of the period (in shares)	759	484
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Weighted-average grant-date fair value, as of the beginning of the period (usd per share)	$ 87.59
Weighted average fair value (usd per share)	152.40
Weighted-average grant-date fair value, vested (usd per share)	87.13
Weighted-average grant-date fair value, cancelled (usd per share)	99.34
Weighted-average grant-date fair value, as of the end of the period (usd per share)	$ 110.50	$ 87.59
Potential awards, percent of target shares, minimum	0.00%
Potential awards, percent of target shares, maximum	200.00%
Restricted stock vested in period, total fair value	$ 23.2	$ 1.3
Tranche one
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Percent of awards in tranche	50.00%
Performance period	1 year
Vesting period	3 years
Tranche two
Restricted stock and Restricted Stock Units, weighted-average grant-date fair value
Percent of awards in tranche	50.00%
Performance period	3 years
Vesting period	3 years