Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Gains (Losses) [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2012	$ 4,576,394	$ 1,439	$ 1,983,957	$ 2,702,915	$ 2,496	$ (114,413)
Beginning Balance, shares at Dec. 31, 2012		143,948,370				4,916,800
Net income	548,686			548,686
Dividends to stockholders	(486,620)			(486,620)
Anti-dilution adjustment paid to stock plan participants ($3.00 per share)	(3,820)			(3,820)
Stock options exercised	110	$ 1	109
Stock options exercised, shares		3,878
Stock-based compensation, net of tax	4,654		4,654
Net gain (loss) on derivative financial instruments	(1,993)				(1,993)
Net loss on investments	(153)				(153)
Ending Balance at Dec. 31, 2013	4,637,258	$ 1,440	1,988,720	2,761,161	350	$ (114,413)
Ending Balance, shares at Dec. 31, 2013		143,952,248				4,916,800
Net income	387,011			387,011
Dividends to stockholders	(481,642)			(481,642)
Anti-dilution adjustment paid to stock plan participants ($3.00 per share)	(4,531)			(4,531)
Treasury stock purchase	$ (87,756)					$ (87,756)
Treasury stock purchase, shares	1,895,561					1,895,561
Stock options exercised	$ 213		213
Stock options exercised, shares		8,012
Stock-based compensation, net of tax	4,965		4,965
Net gain (loss) on derivative financial instruments	(3,861)				(3,861)
Net loss on investments	(94)				(94)
Ending Balance at Dec. 31, 2014	4,451,563	$ 1,440	1,993,898	2,661,999	(3,605)	$ (202,169)
Ending Balance, shares at Dec. 31, 2014		143,960,260				6,812,361
Net income	(274,285)			(274,285)
Dividends to stockholders	(68,578)			(68,578)
Stock-based compensation, net of tax			5,736
Stock-based compensation, vested shares issued (withheld), shares		18,617				7,810
Stock-based compensation, vested shares issued (withheld), value						$ (236)
Stock-based compensation expense, net of shares withheld for taxes	5,500
Net gain (loss) on derivative financial instruments	3,510				3,510
Net loss on investments	(4,940)				(4,940)
Ending Balance at Dec. 31, 2015	$ 4,112,770	$ 1,440	$ 1,999,634	$ 2,319,136	$ (5,035)	$ (202,405)
Ending Balance, shares at Dec. 31, 2015		143,978,877				6,820,171