Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Common Stock	Common Stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional Paid-in Capital	Additional Paid-in Capital Cumulative Effect, Period of Adoption, Adjusted Balance	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance
Beginning balance (in shares) at Dec. 31, 2018				56,615,352	56,615,352
Beginning balance at Dec. 31, 2018	$ 1,576,000	$ (67)	$ 1,575,933	$ 566	$ 566	$ 1,168,442	$ 1,168,442	$ 406,992	$ (67)	$ 406,925
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	40,550							40,550
Shares issued under stock-based compensation programs, net (in shares)				372,344
Shares issued under stock-based compensation programs, net	7,087			$ 4		7,083
Cash dividends declared	(17,019)							(17,019)
Stock dividend declared (in shares)				4,534,908
Stock dividend declared	(96)			$ 45		138,950		(139,091)
Stock-based compensation expense	4,251					4,251
Forfeiture of restricted stock (in shares)				(1,714)
Ending balance (in shares) at Mar. 31, 2019				61,520,890
Ending balance at Mar. 31, 2019	1,610,706			$ 615		1,318,726		291,365
Beginning balance (in shares) at Dec. 31, 2018				56,615,352	56,615,352
Beginning balance at Dec. 31, 2018	1,576,000	(67)	1,575,933	$ 566	$ 566	1,168,442	1,168,442	406,992	(67)	406,925
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	145,723
Ending balance (in shares) at Sep. 30, 2019				62,597,390
Ending balance at Sep. 30, 2019	1,701,801			$ 626		1,341,858		359,317
Beginning balance (in shares) at Mar. 31, 2019				61,520,890
Beginning balance at Mar. 31, 2019	1,610,706			$ 615		1,318,726		291,365
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	54,593							54,593
Shares issued under stock-based compensation programs, net (in shares)				405,094
Shares issued under stock-based compensation programs, net	10,241			$ 4		10,237
Cash dividends declared	(18,521)							(18,521)
Stock-based compensation expense	4,132					4,132
Forfeiture of restricted stock (in shares)				(3,578)
Ending balance (in shares) at Jun. 30, 2019				61,922,406
Ending balance at Jun. 30, 2019	1,661,151			$ 619		1,333,095		327,437
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	50,580							50,580
Shares issued under stock-based compensation programs, net (in shares)				674,984
Shares issued under stock-based compensation programs, net	(1,023)			$ 7		(1,030)
Cash dividends declared	(18,700)							(18,700)
Stock-based compensation expense	9,793					9,793
Ending balance (in shares) at Sep. 30, 2019				62,597,390
Ending balance at Sep. 30, 2019	$ 1,701,801			$ 626		1,341,858		359,317
Beginning balance (in shares) at Dec. 31, 2019	62,574,961			62,574,961	62,574,961
Beginning balance at Dec. 31, 2019	$ 1,782,485	(34)	1,782,451	$ 626	$ 626	1,348,733	1,348,733	433,126	(34)	433,092
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	36,760							36,760
Shares issued under stock-based compensation programs, net (in shares)				477,582
Shares issued under stock-based compensation programs, net	8,194			$ 5		8,189		0
Cash dividends declared	(20,768)							(20,768)
Stock-based compensation expense	4,440					4,440
Forfeiture of restricted stock (in shares)				(48)
Ending balance (in shares) at Mar. 31, 2020				63,052,495
Ending balance at Mar. 31, 2020	$ 1,811,077			$ 631		1,361,362		449,084
Beginning balance (in shares) at Dec. 31, 2019	62,574,961			62,574,961	62,574,961
Beginning balance at Dec. 31, 2019	$ 1,782,485	$ (34)	$ 1,782,451	$ 626	$ 626	1,348,733	$ 1,348,733	433,126	$ (34)	$ 433,092
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	$ 220,094
Ending balance (in shares) at Sep. 30, 2020	64,865,577
Ending balance at Sep. 30, 2020	$ 1,987,999			$ 649		1,397,220		590,130
Beginning balance (in shares) at Mar. 31, 2020				63,052,495
Beginning balance at Mar. 31, 2020	1,811,077			$ 631		1,361,362		449,084
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	84,396			$ 0		0		84,396
Shares issued under stock-based compensation programs, net (in shares)				334,178
Shares issued under stock-based compensation programs, net	(6,862)			$ 3		(6,865)		0
Cash dividends declared	(20,914)							(20,914)
Stock-based compensation expense	5,488					5,488
Forfeiture of restricted stock (in shares)				(1,807)
Ending balance at Jun. 30, 2020	1,873,185			$ 634		1,359,985		512,566
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding				63,384,866
Net Income	98,938							98,938
Shares issued under stock-based compensation programs, net (in shares)				1,480,711
Shares issued under stock-based compensation programs, net	28,642			$ 15		28,627
Cash dividends declared	(21,374)							(21,374)
Stock-based compensation expense	$ 8,608					8,608
Ending balance (in shares) at Sep. 30, 2020	64,865,577
Ending balance at Sep. 30, 2020	$ 1,987,999			$ 649		$ 1,397,220		$ 590,130
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares, Outstanding				64,865,577