CONDENSED CONSOLIDATED STATEMENT OF STOCKHOLDERS' EQUITY - 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, 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)
Purchase of treasury stock (in shares)			0
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, 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
Increase (Decrease) in Stockholders' Equity
Net income	53,910					53,910
Stock amortization and vesting	2,951	$ 65		2,886
Stock amortization and vesting (in shares)		651
Cash dividends	(39,817)					(39,817)
Other comprehensive income (loss)	(136)				(136)
Balance at end of period at Mar. 31, 2020	$ 2,168,395	$ 47,753	$ (1,823,201)	$ 1,785,313	$ 1,224	$ 2,157,306
Balance (in shares) at Mar. 31, 2020		477,533	78,957