Consolidated Balance Sheets (Parenthetical) - USD ($) shares in Thousands, $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Condensed Consolidated Balance Sheets [Abstract]
Allowance for doubtful accounts, accounts receivable	$ 105	$ 105
Accumulated Depreciation Depletion And Amortization Property Plant And Equipment	$ 445,939	$ 432,043
Preferred shares, par value	$ 0.1	$ 0.1
Preferred shares, shares authorized	5,000	5,000
Preferred shares, issued	0	0
Common stock, par value	$ 0.1	$ 0.1
Common stock, shares authorized	145,000	145,000
Common stock, issued	47,334	49,143
Common stock, outstanding	47,334	49,143