Revenue Recognition - Impacts of adoption in the current period - SOCF (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
Net income	$ 10,780	$ 10,909	$ 8,477	$ 22,494
Materials and supplies, prepayments, and other current assets			1,254	(6,586)
Accrued taxes			1,249	(7,876)
Accounts payable and accrued liabilities, advance payments and deposits and other current liabilities			(2,137)	(3,331)
Other assets			(1,208)	2,887
Net Cash Provided by (Used in) Operating Activities			46,037	65,467
Net Cash Provided by (Used in) Investing Activities			(58,838)	(90,033)
Net Cash Provided by (Used in) Financing Activities			(26,784)	(15,488)
Net change in cash, cash equivalents, and restricted cash			(39,763)	$ (39,847)
ASU 2014-09 | Change
CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
Net income	395		813
Materials and supplies, prepayments, and other current assets			681
Accrued taxes			(142)
Accounts payable and accrued liabilities, advance payments and deposits and other current liabilities			(68)
Other assets			342
Net Cash Provided by (Used in) Operating Activities			0
Net Cash Provided by (Used in) Investing Activities			0
Net Cash Provided by (Used in) Financing Activities			0
Net change in cash, cash equivalents, and restricted cash			0
ASU 2014-09 | Under previous guidance
CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
Net income	$ 10,385		7,664
Materials and supplies, prepayments, and other current assets			573
Accrued taxes			1,391
Accounts payable and accrued liabilities, advance payments and deposits and other current liabilities			(2,069)
Other assets			$ (1,550)