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						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 115,506	$ 92,593	$ 113,896	$ 106,053	$ 73,922	$ 90,828	$ 321,995	$ 270,803
Net income attributable to unvested awarded performance stock	(3,772)			(2,716)			(10,386)	(6,185)
Net income attributable to common shares	$ 111,734			$ 103,337			$ 311,609	$ 264,618
Weighted average number of common shares outstanding – basic	282,178			278,132			281,505	276,744
Less unvested awarded performance stock included in weighted average number of common shares outstanding – basic	(9,216)			(7,124)			(9,080)	(6,321)
Weighted average number of common shares outstanding for basic net income per common share	272,962			271,008			272,425	270,423
Dilutive effect of stock options	2,113			4,274			2,104	5,191
Weighted average number of shares outstanding – diluted	275,075			275,282			274,529	275,614
Basic (in dollars per share)	$ 0.41			$ 0.38			$ 1.14	$ 0.98
Diluted (in dollars per share)	$ 0.41			$ 0.38			$ 1.14	$ 0.96