Fair Value Measurements - Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-marketable long-term investments, carrying value	$ 9,000	$ 8,200
Total investments carried at cost	8,487	7,629
CoBank
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total investments carried at cost	$ 7,705	$ 6,818