Supplemental Cash Flow Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Supplemental disclosure:
Interest paid	$ 0	$ 0	$ 0	$ 0
Income taxes paid	4,127	10,329	4,488	10,847
Non-cash investing and financing activities:
Non-cash capital expenditures	(874)	1,047	224	1,312
Dividends declared	$ 6,849	$ 5,834	$ 6,849	$ 5,834