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Investments - Gains and proceeds (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Investments
Realized loss (gain) on sale of investments	$ 0	$ 0
Maturities of investments - debt securities			$ 7,000	$ 7,000
Proceeds from sale of available-for-sale debt securities			1,200	1,200
Purchases of available-for-sale securities			56,900
Maximum
Investments
Realized loss (gain) on sale of investments			$ 100	100
Purchases of available-for-sale securities				$ 100