CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock.	Additional paid-in capital	Retained earnings	Common Stock	Total
Balance at Dec. 31, 2018	$ 8	$ 1,310,648	$ 343,135		$ 1,653,791
Balance (in shares) at Dec. 31, 2018	77,494
Changes in stockholders' equity
Net income			118,831		118,831
Stock based compensation		7,295			7,295
Stock based compensation (in shares)	856
Issuance of common stock in settlement of director fees		136			136
Issuance of common stock in settlement of director fees (in shares)	6
Repurchase of common stock		(1,056)		$ (1,056)	(1,056)
Repurchase of common stock (in shares)	(51)			(51)
Balance at Jun. 30, 2019	$ 8	1,317,023	461,966		1,778,997
Balance (in shares) at Jun. 30, 2019	78,305
Balance at Mar. 31, 2019	$ 8	1,311,914	389,270		1,701,192
Balance (in shares) at Mar. 31, 2019	78,318
Changes in stockholders' equity
Net income			72,696		72,696
Stock based compensation		6,115			6,115
Stock based compensation (in shares)	36
Issuance of common stock in settlement of director fees		50			50
Issuance of common stock in settlement of director fees (in shares)	2
Repurchase of common stock		(1,056)		$ (1,056)	(1,056)
Repurchase of common stock (in shares)	(51)			(51)
Balance at Jun. 30, 2019	$ 8	1,317,023	461,966		1,778,997
Balance (in shares) at Jun. 30, 2019	78,305
Balance at Dec. 31, 2019	$ 8	1,335,107	726,392		2,061,507
Balance (in shares) at Dec. 31, 2019	78,515
Changes in stockholders' equity
Net income			658,920		658,920
Stock based compensation		19,491			19,491
Stock based compensation (in shares)	1,053
Common stock dividends			(19,538)		(19,538)
Issuance of common stock in settlement of director fees		96			96
Issuance of common stock in settlement of director fees (in shares)	3
Repurchase of common stock	$ (1)	(241,282)		$ (241,283)	(241,283)
Repurchase of common stock (in shares)	(7,213)			(7,213)
Balance at Jun. 30, 2020	$ 7	1,113,412	1,365,774		2,479,193
Balance (in shares) at Jun. 30, 2020	72,358
Balance at Mar. 31, 2020	$ 8	1,341,219	1,022,902		2,364,129
Balance (in shares) at Mar. 31, 2020	79,190
Changes in stockholders' equity
Net income			352,677		352,677
Stock based compensation		9,306			9,306
Stock based compensation (in shares)	141
Common stock dividends			(9,805)		(9,805)
Issuance of common stock in settlement of director fees		48			48
Issuance of common stock in settlement of director fees (in shares)	2
Repurchase of common stock	$ (1)	(237,161)		$ (237,162)	(237,162)
Repurchase of common stock (in shares)	(6,975)			(6,975)
Balance at Jun. 30, 2020	$ 7	$ 1,113,412	$ 1,365,774		$ 2,479,193
Balance (in shares) at Jun. 30, 2020	72,358