Fair Value - Quantitative and Qualitative Disclosures about Unobservable Inputs (Details) - Level 3 - USD ($) $ in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 71,114	$ 24,036	$ 11,645
Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	68,109	$ 23,241	11,645	$ 14,065	$ 8,924	$ 12,577
Fair Value, Measurements, Recurring Basis | Non-Binding Broker Quote
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs	$ 12,000
Fair Value, Measurements, Recurring Basis | Non-Binding Broker Quote | Available-for-sale securities
Fair value measurements on a recurring basis, valuation techniques
Fair value, measurement with unobservable inputs			$ 1,300