Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value (Details) - Nonrecurring - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Level 3 | Carrying Value
Financial instruments and fair value measurements
Senior Notes held by SPE	$ 176,837	$ 176,775
Level 3 | Carrying Value | Time deposit
Financial instruments and fair value measurements
Investments held by SPEs	200,000	200,000
Level 3 | Fair Value
Financial instruments and fair value measurements
Senior Notes held by SPE	196,106	193,293
Level 3 | Fair Value | Time deposit
Financial instruments and fair value measurements
Investments held by SPEs	200,000	200,000
Level 1 | Carrying Value | U. S Treasury securities and cash
Financial instruments and fair value measurements
Investments held by SPEs	7,011	7,384
Level 1 | Fair Value | U. S Treasury securities and cash
Financial instruments and fair value measurements
Investments held by SPEs	$ 6,809	$ 7,092