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CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($)
shares in Thousands, $ in Thousands
Common Stock
Retained Earnings
Treasury Stock
Accumulated Other Comprehensive Income (Loss)
Total
Balance at Dec. 31, 2015 $ 641,643 $ 1,275,142 $ (410,090) $ (260) $ 1,506,435
Balance (in shares) at Dec. 31, 2015 79,020   (28,015)    
Increase (Decrease) in Stockholders' Equity          
Net income   (161,586)     (161,586) [1]
Net unrealized appreciation on marketable securities, net of taxes       189 189 [1]
Exercise of common stock options and vested restricted stock units $ 4,979       4,979
Exercise of common stock options and vested restricted stock units (in shares) 609        
Sale of common stock under employee stock purchase plan $ 3,163       3,163
Sale of common stock under employee stock purchase plan (in shares) 152        
Stock based compensation expense related to the issuance of stock options and restricted stock units $ 7,568       7,568
Cash dividends declared (per share)   (9,805)     (9,805)
Balance at Dec. 31, 2016 $ 657,353 1,103,751 $ (410,090) (71) 1,350,943
Balance (in shares) at Dec. 31, 2016 79,781   (28,015)    
Increase (Decrease) in Stockholders' Equity          
Net income   428,907     428,907 [1]
Net unrealized appreciation on marketable securities, net of taxes       21 21 [1]
Exercise of common stock options and vested restricted stock units $ 1,658       1,658
Exercise of common stock options and vested restricted stock units (in shares) 529        
Treasury shares acquired from vested employee stock awards for income tax withholdings     $ (5,080)   (5,080)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)     (145)    
Sale of common stock under employee stock purchase plan $ 3,002       3,002
Sale of common stock under employee stock purchase plan (in shares) 88        
Stock based compensation expense related to the issuance of stock options and restricted stock units $ 10,580       10,580
Impact of adoption of Accounting Standards Update (ASU) 2016-09 (See Note 1)   867     867
Treasury stock purchases     $ (20,008)   (20,008)
Treasury stock purchases (in shares)     (484)    
Cash dividends declared (per share)   (16,568)     (16,568)
Balance at Dec. 31, 2017 $ 672,593 1,516,957 $ (435,178) (50) 1,754,322
Balance (in shares) at Dec. 31, 2017 80,398   (28,644)    
Increase (Decrease) in Stockholders' Equity          
Net income   280,372     280,372
Net unrealized appreciation on marketable securities, net of taxes       18 18
Exercise of common stock options and vested restricted stock units $ 2,174       2,174
Exercise of common stock options and vested restricted stock units (in shares) 780        
Treasury shares acquired from vested employee stock awards for income tax withholdings     $ (13,556)   (13,556)
Treasury shares acquired from vested employee stock awards for income tax withholdings (in shares)     (239)    
Sale of common stock under employee stock purchase plan $ 3,038       3,038
Sale of common stock under employee stock purchase plan (in shares) 61        
Stock based compensation expense related to the issuance of stock options and restricted stock units $ 13,105       13,105
Treasury stock purchases     $ (54,448)   (54,448)
Treasury stock purchases (in shares)     (968)    
Cash dividends declared (per share)   (20,744)     (20,744)
Balance at Dec. 31, 2018 $ 690,910 $ 1,776,585 $ (503,182) $ (32) $ 1,964,281
Balance (in shares) at Dec. 31, 2018 81,239   (29,851)    
[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements of this report for additional information.