Financial Instruments and Fair Value Measurements - Carrying Amount and Fair Value (Details) - Nonrecurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Carrying Value
Financial instruments and fair value measurements
Debt	$ 94,507	$ 70,976
Fair Value
Financial instruments and fair value measurements
Debt	96,814	69,956
Level 1 | Carrying Value | U. S Treasury Bills and cash
Financial instruments and fair value measurements
Investments held by SPEs	6,771	7,384
Level 1 | Fair Value | U. S Treasury Bills and cash
Financial instruments and fair value measurements
Investments held by SPEs	6,712	7,092
Level 2 | Carrying Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	89,969	68,146
Level 2 | Carrying Value | Variable rate debt
Financial instruments and fair value measurements
Debt	4,538	2,830
Level 2 | Fair Value | Fixed-rate debt
Financial instruments and fair value measurements
Debt	92,276	67,126
Level 2 | Fair Value | Variable rate debt
Financial instruments and fair value measurements
Debt	4,538	2,830
Level 3 | Carrying Value
Financial instruments and fair value measurements
Senior Notes held by SPE	177,026	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	204,347	193,293
Level 3 | Fair Value | Time deposit
Financial instruments and fair value measurements
Investments held by SPEs	$ 200,000	$ 200,000