CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Treasury Stock	Additional Paid-in Capital	Retained Earnings	AOCI Attributable to Parent
Beginning balance at Dec. 31, 2017	$ 5,774,272	$ 2,481	$ (599)	$ 5,577,732	$ 195,990	$ (1,332)
Beginning balance Shares at Dec. 31, 2017		248,144,000
Issuance of common stock	(1,217)	$ 11		(1,228)
Issuance of common stock, shares		1,092,000
Stock-based compensation expense	17,201			17,201
Cash dividends paid ($0.02 per share)	(4,971)				(4,971)
Treasury stock issuance			30	(30)
Other comprehensive income	69					69
Net income	49,238				49,238
Ending balance at Mar. 31, 2018	5,834,592	$ 2,492	(569)	5,593,675	240,257	(1,263)
Ending balance Shares at Mar. 31, 2018		249,236,000
Beginning balance at Dec. 31, 2017	5,774,272	$ 2,481	(599)	5,577,732	195,990	(1,332)
Beginning balance Shares at Dec. 31, 2017		248,144,000
Ending balance at Dec. 31, 2018	$ 4,059,431	$ 2,495	(391)	5,628,447	(1,570,462)	(658)
Ending balance Shares at Dec. 31, 2018	249,519,687	249,520,000
Issuance of common stock	$ (2,379)	$ 17		(2,396)
Issuance of common stock, shares		1,628,000
Issuance of common stock upon vesting of PSUs				4	(4)
Stock-based compensation expense	9,155			9,155
Cash dividends paid ($0.02 per share)	(5,023)				(5,023)
Other comprehensive income	60					60
Net income	1,419				1,419
Ending balance at Mar. 31, 2019	$ 4,062,663	$ 2,512	$ (391)	$ 5,635,210	$ (1,574,070)	$ (598)
Ending balance Shares at Mar. 31, 2019	251,147,799	251,148,000