CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Treasury Stock	Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Balance at beginning of period at Dec. 31, 2017	$ 2,523,905	$ 47,555	$ (430,576)	$ 1,742,419	$ 2,077	$ 1,162,430
Balance (in shares) at Dec. 31, 2017		475,547	14,936
Increase (Decrease) in Stockholders' Equity
Net income	117,231					117,231
Stock amortization and vesting	302	$ 53		249
Stock amortization and vesting (in shares)		534
Purchase of treasury stock	(207,135)		$ (207,135)
Purchase of treasury stock (in shares)			8,328
Cash dividends	(27,647)					(27,647)
Other comprehensive income (loss)	306				306
Balance at end of period at Mar. 31, 2018	2,406,516	$ 47,608	$ (637,711)	1,742,668	2,383	1,251,568
Balance (in shares) at Mar. 31, 2018		476,081	23,264
Balance at beginning of period at Dec. 31, 2017	2,523,905	$ 47,555	$ (430,576)	1,742,419	2,077	1,162,430
Balance (in shares) at Dec. 31, 2017		475,547	14,936
Increase (Decrease) in Stockholders' Equity
Net income	281,998
Balance at end of period at Sep. 30, 2018	2,094,147	$ 47,609	$ (1,074,708)	1,756,337	2,112	1,362,797
Balance (in shares) at Sep. 30, 2018		476,089	42,080
Increase (Decrease) in Stockholders' Equity
Cumulative impact from accounting change	(446)					(446)
Balance at beginning of period at Mar. 31, 2018	2,406,516	$ 47,608	$ (637,711)	1,742,668	2,383	1,251,568
Balance (in shares) at Mar. 31, 2018		476,081	23,264
Increase (Decrease) in Stockholders' Equity
Net income	42,431					42,431
Exercise of stock appreciation rights	1	$ 1
Exercise of stock appreciation rights (in shares)		3
Stock amortization and vesting	6,769	$ 0		6,769
Stock amortization and vesting (in shares)		2
Purchase of treasury stock	(274,337)		$ (274,337)
Purchase of treasury stock (in shares)			11,646
Cash dividends	(27,071)					(27,071)
Other comprehensive income (loss)	(135)				(135)
Balance at end of period at Jun. 30, 2018	2,154,174	$ 47,609	$ (912,048)	1,749,437	2,248	1,266,928
Balance (in shares) at Jun. 30, 2018		476,086	34,910
Increase (Decrease) in Stockholders' Equity
Net income	122,337					122,337
Exercise of stock appreciation rights	0
Exercise of stock appreciation rights (in shares)		2
Stock amortization and vesting	6,900			6,900
Stock amortization and vesting (in shares)		1
Purchase of treasury stock	(162,660)		$ (162,660)
Purchase of treasury stock (in shares)			7,170
Cash dividends	(26,468)					(26,468)
Other comprehensive income (loss)	(136)				(136)
Balance at end of period at Sep. 30, 2018	2,094,147	$ 47,609	$ (1,074,708)	1,756,337	2,112	1,362,797
Balance (in shares) at Sep. 30, 2018		476,089	42,080
Balance at beginning of period at Dec. 31, 2018	2,088,159	$ 47,610	$ (1,334,688)	1,763,142	4,437	1,607,658
Balance (in shares) at Dec. 31, 2018		476,095	53,410
Increase (Decrease) in Stockholders' Equity
Net income	262,763					262,763
Stock amortization and vesting	(213)	$ 68		(281)
Stock amortization and vesting (in shares)		682
Purchase of treasury stock	(28)		$ (28)
Cash dividends	(29,605)					(29,605)
Other comprehensive income (loss)	(137)				(137)
Balance at end of period at Mar. 31, 2019	2,320,939	$ 47,678	$ (1,334,716)	1,762,861	4,300	1,840,816
Balance (in shares) at Mar. 31, 2019		476,777	53,410
Balance at beginning of period at Dec. 31, 2018	2,088,159	$ 47,610	$ (1,334,688)	1,763,142	4,437	1,607,658
Balance (in shares) at Dec. 31, 2018		476,095	53,410
Increase (Decrease) in Stockholders' Equity
Net income	534,130
Purchase of treasury stock	$ (316,100)
Purchase of treasury stock (in shares)	15,500
Balance at end of period at Sep. 30, 2019	$ 2,213,576	$ 47,688	$ (1,650,784)	1,775,579	4,027	2,037,066
Balance (in shares) at Sep. 30, 2019		476,882	68,957
Balance at beginning of period at Mar. 31, 2019	2,320,939	$ 47,678	$ (1,334,716)	1,762,861	4,300	1,840,816
Balance (in shares) at Mar. 31, 2019		476,777	53,410
Increase (Decrease) in Stockholders' Equity
Net income	181,009					181,009
Stock amortization and vesting	6,344	$ 10		6,334
Stock amortization and vesting (in shares)		102
Purchase of treasury stock	(125,260)		$ (125,260)
Purchase of treasury stock (in shares)			5,081
Cash dividends	(38,092)					(38,092)
Other comprehensive income (loss)	(136)				(136)
Balance at end of period at Jun. 30, 2019	2,344,804	$ 47,688	$ (1,459,976)	1,769,195	4,164	1,983,733
Balance (in shares) at Jun. 30, 2019		476,879	58,491
Increase (Decrease) in Stockholders' Equity
Net income	90,358					90,358
Stock amortization and vesting	6,384	$ 3		6,384
Purchase of treasury stock	(190,808)		$ (190,808)
Purchase of treasury stock (in shares)			10,466
Cash dividends	(37,025)					(37,025)
Other comprehensive income (loss)	(137)				(137)
Balance at end of period at Sep. 30, 2019	$ 2,213,576	$ 47,688	$ (1,650,784)	$ 1,775,579	$ 4,027	$ 2,037,066
Balance (in shares) at Sep. 30, 2019		476,882	68,957