Summary of Significant Accounting Policies - Fair Value of Financial Instruments Measured on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 16,743	$ 10,380	$ 6,449	$ 11,012
Restricted cash for Development Projects	22,400	41,900
Short-term Investments	7,000	94,500
Long-term Debt	488,887	469,736
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, at Carrying Value	16,743	10,380
Cash and Cash Equivalents, Fair Value Disclosure	16,743	10,380
Restricted cash	984	693
Restricted Cash, Current, Fair Value Disclosure	984	693
Restricted cash for Development Projects	22,414	41,982
Restricted Cash, Noncurrent, Fair Value Disclosure	22,414	41,982
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term Investments	7,044	94,449
Short-term Investments, Fair Value Disclosure	7,026	94,209
Bangkok Bank Loan [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Lines of Credit, Fair Value Disclosure	16,000	16,000
Long-term Line of Credit	16,000	16,000
Fifth Third Revolver [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Lines of Credit, Fair Value Disclosure	15	0
Long-term Line of Credit	15,000	0
Equipment Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	27,700
Equipment Loans [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	27,682	31,095
Lines of Credit, Fair Value Disclosure	27,682	31,095
Interest Rate Cap [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	282	251
Notes Payable to Banks [Member] | Term Loan Facility, Term B Loan [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt	442,474	443,161
Notes Payable, Fair Value Disclosure	448,589	449,749
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term Investments	3,500	59,400
US Treasury Notes Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short-term Investments	$ 3,500	$ 35,100