CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY Statement - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Loss [Member]
Balance, shares at Dec. 31, 2016		51,958		3,626
Balance, beginning of period at Dec. 31, 2016	$ 1,676,227	$ 520	$ 1,384,271	$ (95,769)	$ 396,919	$ (9,714)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	56,567	0	0	0	56,567	0
Foreign currency translation adjustment	51	0	0	0	0	51
Reclassification of net losses on derivative instruments from OCI to net income, net of tax	93	$ 0	0	$ 0	0	93
Stock based compensation, shares		127		15
Stock based compensation, value	1,459	$ 1	2,929	$ (1,471)	0	0
Dividends on common stock	$ (25,194)	0	0	0	(25,194)	0
Dividends per share	$ 0.525
Balance, end of period at Mar. 31, 2017	$ 1,709,203	$ 521	1,387,200	$ (97,240)	428,292	(9,570)
Balance, shares at Mar. 31, 2017		52,085		3,641
Balance, shares at Dec. 31, 2017		52,981		3,609
Balance, beginning of period at Dec. 31, 2017	1,798,915	$ 530	1,445,181	$ (96,376)	458,352	(8,772)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	58,499	0	0	0	58,499	0
Foreign currency translation adjustment	95	0	0	0	0	95
Reclassification of net losses on derivative instruments from OCI to net income, net of tax	113	$ 0	0	$ 0	0	113
Stock based compensation, shares		71		(30)
Stock based compensation, value	1,817	$ 1	2,485	$ (669)	0	0
Issuance of shares		0
Issuance of shares, value	2,129	$ 0	956	$ 1,173	0	0
Issuance of shares, treasury stock				0
Dividends on common stock	$ (26,945)	0	0	$ 0	(26,945)	0
Dividends per share	$ 0.550
Balance, end of period at Mar. 31, 2018	$ 1,834,623	$ 531	1,448,622	$ (95,872)	492,049	(10,707)
Balance, shares at Mar. 31, 2018		53,052		3,579
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of certain tax effects from AOCL	$ 0	$ 0	$ 0	$ 0	$ (2,143)	$ 2,143