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			240,304		240,304
Stock based compensation		18,390			18,390
Stock based compensation (in shares)	984
Issuance of common stock in settlement of director fees		184			184
Issuance of common stock in settlement of director fees (in shares)	8
Repurchase of common stock		(1,056)		$ (1,056)	(1,056)
Repurchase of common stock (in shares)	(51)			(51)
Balance at Sep. 30, 2019	$ 8	1,328,166	583,439		1,911,613
Balance (in shares) at Sep. 30, 2019	78,435
Balance at Jun. 30, 2019	$ 8	1,317,023	461,966		1,778,997
Balance (in shares) at Jun. 30, 2019	78,305
Changes in stockholders' equity
Net income			121,473		121,473
Stock based compensation		11,095			11,095
Stock based compensation (in shares)	128
Issuance of common stock in settlement of director fees		48			48
Issuance of common stock in settlement of director fees (in shares)	2
Balance at Sep. 30, 2019	$ 8	1,328,166	583,439		1,911,613
Balance (in shares) at Sep. 30, 2019	78,435
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			1,194,080		1,194,080
Stock based compensation		29,386			29,386
Stock based compensation (in shares)	1,212
Common stock dividends			(19,830)		(19,830)
Issuance of common stock in settlement of director fees		144			144
Issuance of common stock in settlement of director fees (in shares)	4
Repurchase of common stock	$ (1)	(248,209)		$ (248,210)	(248,210)
Repurchase of common stock (in shares)	(7,331)			(7,331)
Balance at Sep. 30, 2020	$ 7	1,116,428	1,900,642		3,017,077
Balance (in shares) at Sep. 30, 2020	72,400
Balance at Jun. 30, 2020	$ 7	1,113,412	1,365,774		2,479,193
Balance (in shares) at Jun. 30, 2020	72,358
Changes in stockholders' equity
Net income			535,160		535,160
Stock based compensation		9,895			9,895
Stock based compensation (in shares)	159
Common stock dividends			(292)		(292)
Issuance of common stock in settlement of director fees		48			48
Issuance of common stock in settlement of director fees (in shares)	1
Repurchase of common stock		(6,927)		$ (6,927)	(6,927)
Repurchase of common stock (in shares)	(118)			(118)
Balance at Sep. 30, 2020	$ 7	$ 1,116,428	$ 1,900,642		$ 3,017,077
Balance (in shares) at Sep. 30, 2020	72,400