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Stockholders' Equity - Shares Repurchased (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Equity [Abstract]      
Number of shares repurchased 2,317,617 3,979,170 525,504
Average price per share (in dollars per share) $ 127.04 $ 109.64 $ 85.27
Total cost of repurchases $ 294,467 $ 436,369 $ 44,822