Net Income Per Share - Reconciliation between Basic and Diluted Weighted Average Shares Outstanding (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income	$ 96,784	$ 152,400	$ 92,593	$ 113,896	$ 249,184	$ 206,489
Net income attributable to unvested awarded performance stock	(3,580)		(3,020)		(8,855)	(6,273)
Net income attributable to common shares	$ 93,204		$ 89,573		$ 240,329	$ 200,216
Weighted average number of common shares outstanding – basic	283,591		281,754		282,994	281,163
Less unvested awarded performance stock included in weighted average number of common shares outstanding – basic	(10,491)		(9,191)		(10,056)	(8,542)
Weighted average number of common shares outstanding for basic net income per common share	273,100		272,563		272,938	272,621
Dilutive effect of stock options	1,262		1,839		1,581	2,083
Weighted average number of shares outstanding – diluted	274,362		274,402		274,519	274,704
Basic (in dollars per share)	$ 0.34		$ 0.33		$ 0.88	$ 0.73
Diluted (in dollars per share)	$ 0.34		$ 0.33		$ 0.88	$ 0.73