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, 2015	$ 935,654	$ 805	$ 102,734	$ 1,022,966	$ (13,063)	$ (177,788)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	75,275	0	0	79,129	(3,854)	0
Dividends on common stock	(17,299)	0	0	(17,299)	0	0
Equity compensation activity, including excess tax benefits	(1,224)	0	(4,080)	0	0	2,856
Non-cash equity compensation expense	2,381	0	2,381	0	0	0
BALANCE at Dec. 31, 2016	994,787	805	101,035	1,084,796	(16,917)	(174,932)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	203,971	0	0	202,889	1,082	0
Dividends on common stock	(19,523)	0	0	(19,523)	0	0
Equity compensation activity, including excess tax benefits	829	0	(3,481)	0	0	4,310
Non-cash equity compensation expense	4,546	0	4,546	0	0	0
Cumulative effect of accounting change	172	0	463	(291)	0	0
BALANCE at Dec. 31, 2017	1,184,782	805	102,563	1,267,871	(15,835)	(170,622)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income	167,910	0	0	168,148	(238)	0
Purchases of common stock	(72,165)	0	0	0	0	(72,165)
Dividends on common stock	(24,284)	0	0	(24,284)	0	0
Equity compensation activity, including excess tax benefits	(895)	0	(2,502)	0	0	1,607
Non-cash equity compensation expense	7,394	0	7,394	0	0	0
Cumulative effect of accounting change	2,011	0	0	2,011	0	0
BALANCE at Dec. 31, 2018	$ 1,264,753	$ 805	$ 107,455	$ 1,413,746	$ (16,073)	$ (241,180)