Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value (Details) - Nonrecurring - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Carrying Value
Financial instruments and fair value measurements
Debt	$ 185,957	$ 161,418
Fair Value
Financial instruments and fair value measurements
Debt	190,743	163,802
Level 1 | Carrying Value | U.S. Treasury Bills
Financial instruments and fair value measurements
Investments held by SPEs	5,428	5,759
Level 1 | Fair Value | U.S. Treasury Bills
Financial instruments and fair value measurements
Investments held by SPEs	5,869	6,363
Level 2 | Carrying Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	121,273	114,125
Level 2 | Carrying Value | Variable rate debt
Financial instruments and fair value measurements
Debt	64,684	47,293
Level 2 | Fair Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	126,059	116,509
Level 2 | Fair Value | Variable rate debt
Financial instruments and fair value measurements
Debt	64,684	47,293
Level 3 | Carrying Value
Financial instruments and fair value measurements
Senior Notes held by SPE	177,426	177,289
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	208,545	216,363
Level 3 | Fair Value | Time deposit
Financial instruments and fair value measurements
Investments held by SPEs	$ 200,000	$ 200,000