Significant Accounting Policies - Reconciliation of Components of Basic and Diluted Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share Reconciliation [Abstract]
Net income	$ 40,674	$ 47,493
Weighted average common shares outstanding - basic	197,383	196,550
Effect of dilutive instruments	418
Weighted average common shares outstanding - diluted	197,801	196,550
Net income per common share - basic	$ 0.21	$ 0.24
Net income per common share - fully diluted	$ 0.21	$ 0.24