Revenue Recognition - Impacts of adoption in the current period - SOCF (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
Net income (loss)	$ (2,303)	$ 11,584
Materials and supplies, prepayments, and other current assets	3,991	(1,789)
Accrued taxes	3,292	4,341
Accounts payable and accrued liabilities, advance payments and deposits and other current liabilities	(2,306)	(14,465)
Other assets	(581)	2,033
Net Cash Provided by (Used in) Operating Activities	22,544	32,089
Net Cash Provided by (Used in) Investing Activities	(7,077)	(22,622)
Net Cash Provided by (Used in) Financing Activities	(18,130)	(15,628)
Net change in cash, cash equivalents, and restricted cash	(2,694)	$ (5,954)
ASU 2014-09 | Change
CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS
Net income (loss)	418
Materials and supplies, prepayments, and other current assets	(350)
Accrued taxes	93
Accounts payable and accrued liabilities, advance payments and deposits and other current liabilities	46
Other assets	(207)
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 (loss)	(2,721)
Materials and supplies, prepayments, and other current assets	4,341
Accrued taxes	3,199
Accounts payable and accrued liabilities, advance payments and deposits and other current liabilities	(2,352)
Other assets	$ (374)