Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value (Details) - Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Carrying Value
Financial instruments and fair value measurements
Debt	$ 161,418	$ 94,507
Fair Value
Financial instruments and fair value measurements
Debt	163,802	96,814
Level 1 | Carrying Value | U. S Treasury Bills and cash
Financial instruments and fair value measurements
Investments held by SPEs	5,759	6,382
Level 1 | Fair Value | U. S Treasury Bills and cash
Financial instruments and fair value measurements
Investments held by SPEs	6,363	6,712
Level 2 | Carrying Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	114,125	89,969
Level 2 | Carrying Value | Variable rate debt
Financial instruments and fair value measurements
Debt	47,293	4,538
Level 2 | Fair Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	116,509	92,276
Level 2 | Fair Value | Variable rate debt
Financial instruments and fair value measurements
Debt	47,293	4,538
Level 3 | Carrying Value
Financial instruments and fair value measurements
Senior Notes held by SPE	177,289	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	216,363	204,347
Level 3 | Fair Value | Time deposit
Financial instruments and fair value measurements
Investments held by SPEs	$ 200,000	$ 200,000