Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Carrying Value
Financial instruments and fair value measurements
Debt	$ 122,980	$ 94,507
Fair Value
Financial instruments and fair value measurements
Debt	131,089	96,814
Level 1 | Carrying Value | U. S Treasury Bills and cash
Financial instruments and fair value measurements
Investments held by SPEs	6,438	6,771
Level 1 | Fair Value | U. S Treasury Bills and cash
Financial instruments and fair value measurements
Investments held by SPEs	6,731	6,712
Level 2 | Carrying Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	101,123	89,969
Level 2 | Carrying Value | Variable rate debt
Financial instruments and fair value measurements
Debt	21,857	4,538
Level 2 | Fair Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	109,232	92,276
Level 2 | Fair Value | Variable rate debt
Financial instruments and fair value measurements
Debt	21,857	4,538
Level 3 | Carrying Value
Financial instruments and fair value measurements
Senior Notes held by SPE	177,156	177,026
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	222,866	204,347
Level 3 | Fair Value | Time deposit
Financial instruments and fair value measurements
Investments held by SPEs	$ 200,000	$ 200,000