Components of Calculation of Basic and Diluted Earnings Per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended		9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income (loss)	$ 54,362	$ (38,697)	$ 16,276	$ 10,295	$ 51,625	$ (2,521)	$ 67,290	$ 24,050
Weighted-average shares – basic	15,110		15,062				15,105	15,054
Effect of dilutive securities	50		282				60	224
Weighted-average shares – diluted	15,160		15,344				15,165	15,278