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Stockholders' Equity - ESPP Activity (Details) - $ / shares
shares in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Equity [Abstract]      
Number of shares issued (in thousands) 164 202 197
Weighted average price $ 96.95 $ 59.93 $ 48.65