CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - 6 months ended Jun. 30, 2018 - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income
Balance at beginning of period (shares) at Dec. 31, 2017		95,437
Balance at beginning of period at Dec. 31, 2017	$ 2,568,278	$ 954	$ 2,764,384	$ (199,259)	$ 2,199
Increase (Decrease) in Stockholders' Equity
Dividends paid on stock awards subsequently forfeited	46		29	17
Dividends	(15,262)			(15,262)
Dividends in excess of retained earnings	(15,250)		(15,250)
Net income	327,315			327,315
Unrealized change in fair value of investments, net of tax	2				2
Issuance of restricted stock awards (shares)		29
Common stock reacquired and retired (shares)		(8)
Common stock reacquired and retired	(946)		(946)
Restricted stock forfeited and retired (shares)		(82)
Exercise of stock options (shares)		17
Exercise of stock options	1,249		1,249
Stock-based compensation	21,066		21,066
Balance at end of period (shares) at Jun. 30, 2018		95,393
Balance at end of period at Jun. 30, 2018	$ 2,886,498	$ 954	$ 2,770,532	$ 112,811	$ 2,201