CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Total	Common Shares	Treasury Stock	Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings
Balance at beginning of period at Dec. 31, 2016	$ 2,567,667	$ 47,504	$ (306,835)	$ 1,727,310	$ 985	$ 1,098,703
Balance (in shares) at Dec. 31, 2016		475,043	9,893
Increase (Decrease) in Stockholders' Equity
Net income	100,393					100,393
Exercise of stock appreciation rights	0	$ 14		(14)
Exercise of stock appreciation rights (in shares)		137
Stock amortization and vesting	15,160	$ 37		15,123
Stock amortization and vesting (in shares)		367
Purchase of treasury stock	(123,741)		$ (123,741)
Purchase of treasury stock (in shares)			5,043
Cash dividends	(78,838)					(78,838)
Other comprehensive income	1,092				1,092
Balance at end 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	557,043					557,043
Exercise of stock appreciation rights	0	$ 1		(1)
Exercise of stock appreciation rights (in shares)		9
Stock amortization and vesting	20,778	$ 54		20,724
Stock amortization and vesting (in shares)		539
Purchase of treasury stock	(904,112)		$ (904,112)
Purchase of treasury stock (in shares)			38,474
Cash dividends	(111,369)					(111,369)
Other comprehensive income	2,360				2,360
Balance at end 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	681,070					681,070
Stock amortization and vesting	19,363	$ 78		19,285
Stock amortization and vesting (in shares)		787
Purchase of treasury stock	(488,513)		$ (488,513)
Purchase of treasury stock (in shares)			25,547
Cash dividends	(145,515)					(145,515)
Other comprehensive income	(3,077)				(3,077)
Balance at end of period at Dec. 31, 2019	$ 2,151,487	$ 47,688	$ (1,823,201)	$ 1,782,427	$ 1,360	$ 2,143,213
Balance (in shares) at Dec. 31, 2019		476,882	78,957