Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock
BALANCE at Dec. 31, 2017	$ 1,184,782	$ 805	$ 102,563	$ 1,267,871	$ (15,835)	$ (170,622)
Comprehensive income	30,410	0	0	27,807	2,603	0
Dividends on common stock	(5,071)	0	0	(5,071)	0	0
Equity compensation activity	(438)	0	(1,069)	0	0	631
Non-cash equity compensation expense	1,410	0	1,410	0	0	0
Cumulative effect of accounting change	2,011	0	0	2,011	0	0
BALANCE at Mar. 31, 2018	1,213,104	805	102,904	1,292,618	(13,232)	(169,991)
BALANCE at Dec. 31, 2017	1,184,782	805	102,563	1,267,871	(15,835)	(170,622)
Comprehensive income	115,761
BALANCE at Sep. 30, 2018	1,243,501	805	106,904	1,365,523	(13,659)	(216,072)
BALANCE at Mar. 31, 2018	1,213,104	805	102,904	1,292,618	(13,232)	(169,991)
Comprehensive income	35,821	0	0	38,264	(2,443)	0
Dividends on common stock	(6,466)	0	0	(6,466)	0	0
Purchases of common stock	(22,884)	0	0	0	0	(22,884)
Equity compensation activity	7	0	(84)	0	0	91
Non-cash equity compensation expense	1,979	0	1,979	0	0	0
BALANCE at Jun. 30, 2018	1,221,561	805	104,799	1,324,416	(15,675)	(192,784)
Comprehensive income	49,530	0	0	47,514	2,016	0
Dividends on common stock	(6,407)	0	0	(6,407)	0	0
Purchases of common stock	(23,529)	0	0	0	0	(23,529)
Equity compensation activity	228	0	(13)	0	0	241
Non-cash equity compensation expense	2,118	0	2,118	0	0	0
BALANCE at Sep. 30, 2018	1,243,501	805	106,904	1,365,523	(13,659)	(216,072)
BALANCE at Dec. 31, 2018	1,264,753	805	107,455	1,413,746	(16,073)	(241,180)
Comprehensive income	36,599	0	0	36,086	513	0
Dividends on common stock	(6,290)	0	0	(6,290)	0	0
Purchases of common stock	(20,545)	0	0	0	0	(20,545)
Equity compensation activity	(1,179)	0	(1,578)	0	0	399
Non-cash equity compensation expense	2,051	0	2,051	0	0	0
BALANCE at Mar. 31, 2019	1,275,389	805	107,928	1,443,542	(15,560)	(261,326)
BALANCE at Dec. 31, 2018	1,264,753	805	107,455	1,413,746	(16,073)	(241,180)
Comprehensive income	116,876
BALANCE at Sep. 30, 2019	1,064,804	805	112,064	1,252,344	(17,645)	(282,764)
BALANCE at Mar. 31, 2019	1,275,389	805	107,928	1,443,542	(15,560)	(261,326)
Comprehensive income	43,738	0	0	43,318	420	0
Dividends on common stock	(267,331)	0	0	(267,331)	0	0
Purchases of common stock	(21,756)	0	0	0	0	(21,756)
Equity compensation activity	(10)	0	(140)	0	0	130
Non-cash equity compensation expense	2,314	0	2,314	0	0	0
BALANCE at Jun. 30, 2019	1,032,344	805	110,102	1,219,529	(15,140)	(282,952)
Comprehensive income	36,539	0	0	39,044	(2,505)	0
Dividends on common stock	(6,229)	0	0	(6,229)	0	0
Equity compensation activity	169	0	(19)	0	0	188
Non-cash equity compensation expense	1,981	0	1,981	0	0	0
BALANCE at Sep. 30, 2019	$ 1,064,804	$ 805	$ 112,064	$ 1,252,344	$ (17,645)	$ (282,764)