Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Level 3 | Carrying Value
Financial instruments and fair value measurements
Senior Notes held by SPE	$ 176,655	$ 176,537
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	190,282	198,530
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 equivalents
Financial instruments and fair value measurements
Investments held by SPEs	7,664	7,989
Level 1 | Fair Value | U. S Treasury securities and cash equivalents
Financial instruments and fair value measurements
Investments held by SPEs	$ 7,304	$ 7,797