CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative impact from accounting change	Common Shares	Treasury Stock	Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Retained Earnings Cumulative impact from accounting change
Balance (in shares) at Dec. 31, 2017			475,547,000	14,936,000
Balance at beginning of period at Dec. 31, 2017	$ 2,523,905	$ (446)	$ 47,555	$ (430,576)	$ 1,742,419	$ 2,077	$ 1,162,430	$ (446)
Increase (Decrease) in Stockholders' Equity
Net income	557,043						557,043
Exercise of stock appreciation rights (in shares)			9,000
Exercise of stock appreciation rights	0		$ 1		(1)
Stock amortization and vesting (in shares)			539,000
Stock amortization and vesting	20,778		$ 54		20,724
Purchase of treasury stock (in shares)				38,474,000
Purchase of treasury stock	(904,112)			$ (904,112)
Cash dividends	(111,369)						(111,369)
Other comprehensive income	2,360					2,360
Balance (in shares) at Dec. 31, 2018			476,095,000	53,410,000
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
Increase (Decrease) in Stockholders' Equity
Net income	681,070						681,070
Stock amortization and vesting (in shares)			787,000
Stock amortization and vesting	19,363		$ 78		19,285
Purchase of treasury stock (in shares)				25,547,000
Purchase of treasury stock	(488,513)			$ (488,513)
Cash dividends	(145,515)						(145,515)
Other comprehensive income	(3,077)					(3,077)
Balance (in shares) at Dec. 31, 2019			476,882,000	78,957,000
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
Increase (Decrease) in Stockholders' Equity
Net income	200,529						200,529
Stock amortization and vesting (in shares)			947,000
Stock amortization and vesting	22,022		$ 95		21,927
Cash dividends	(159,390)						(159,390)
Other comprehensive income	1,059					1,059
Balance (in shares) at Dec. 31, 2020			477,829,000	78,957,000
Balance at end of period at Dec. 31, 2020	$ 2,215,707		$ 47,783	$ (1,823,201)	$ 1,804,354	$ 2,419	$ 2,184,352