Investments, Gains (Losses) on Investments (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gains (Losses) on Investments [Abstract]
Impairment losses on investments										$ (2,861)	$ (497)	$ (19)
Change in value of limited partnership investments										(1,442)	1,644	(9,343)
Gains (losses) on equity securities [Abstract]
Realized gains (losses) on equity securities sold during the period	[1]									(8,058)	1,938	(3,072)
Unrealized gains (losses) on equity securities held at the end of the period										1,943	9,287	(9,697)
Total realized and unrealized gains (losses) on equity securities										(6,115)	11,225	(12,769)
Net realized and unrealized gains (losses) on investments		$ 7,761	$ 144	$ 10,615	$ (27,756)	$ 3,848	$ 125	$ 2,889	$ 6,027	(9,236)	12,889	(25,691)
Proceeds from sales of equity securities										51,713	21,621	149,195
Realized gains on equity securities												51,900
Recorded write down to earnings										1,826
Allowance for credit losses		$ 1,035								1,035
Fixed Income Securities [Member]
Gains (Losses) on Investments [Abstract]
Gross gains on available-for-sale investments during the period										11,125	11,009	10,807
Gross losses on available-for-sale investments during the period										$ (9,943)	$ (10,492)	$ (14,367)

[1] During 2018, the Company sold $149,195 in equity securities, resulting in a gain on sale of $51,900.  The majority of these gains were included in unrealized gains within other comprehensive income (loss) at December 31, 2017 and, as a result of the adoption of ASU 2016-01, were reclassified to retained earnings as of January 1, 2018 and were therefore not recognized in the consolidated statements of operations for the year ended December 31, 2018.