Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Stockholders equity, beginning of period at Dec. 31, 2017		$ 1,476	$ 1,127,137	$ (3,252)	$ 815,075	$ 0
Changes in Stockholders' Equity
Issuance of management incentive shares		6	(6)
Cancellation of treasury stock		(10)	(31,404)			31,414
Stock-based compensation expense			15,073
Other comprehensive income (loss)	$ (25,741)			(25,741)
Net income	467,363				467,363
Treasury stock acquired						(31,414)
Stockholders equity, end of period at Dec. 31, 2018	2,365,717	1,472	1,110,800	(28,993)	1,282,438	0
Changes in Stockholders' Equity
Issuance of management incentive shares		7	(7)
Cancellation of treasury stock		(3)	(9,002)			9,005
Dividends and dividend equivalents declared			276		(29,624)
Stock-based compensation expense			16,588
Other comprehensive income (loss)	85,180			85,180
Net income	555,713				555,713
Treasury stock acquired						(9,005)
Stockholders equity, end of period at Dec. 31, 2019	2,984,845	1,476	1,118,655	56,187	1,808,527	0
Changes in Stockholders' Equity
Issuance of common shares		207	439,755
Issuance of management incentive shares		5	(5)
Cancellation of treasury stock		(2)	(6,352)			6,354
Dividends and dividend equivalents declared			648		(70,058)
Stock-based compensation expense			18,462
Other comprehensive income (loss)	82,087			82,087
Net income	413,041				413,041
Treasury stock acquired						(6,354)
Stockholders equity, end of period at Dec. 31, 2020	$ 3,862,633	$ 1,686	$ 1,571,163	$ 138,274	$ 2,151,510	$ 0