Investments, Net Realized and Unrealized Gains (Losses) on Investments (Details) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($) Security	Mar. 31, 2019 USD ($)
Components of net realized and unrealized gains (losses) on investments [Abstract]
Gross gains on available-for-sale fixed income securities during the period	$ 2,652	$ 3,171
Gross losses on available-for-sale fixed income securities sold during the period	(4,741)	(3,527)
Impairment losses on investments	(40)	(260)
Change in value of limited partnership investments	(676)	408
Gains (losses) on equity securities [Abstract]
Realized gains (losses) on equity securities sold during the period	(2,698)	317
Unrealized gains (losses) on equity securities held at the end of the period	(22,253)	5,919
Realized and unrealized gains (losses) on equity securities during the period	(24,951)	6,236
Net realized and unrealized gains (losses) on investments	$ (27,756)	$ 6,028
Number of security intended to sell in unrealized loss position \| Security	1
Recorded write down to earnings	$ 40
Undistributed earnings, limited partnership	$ 4,453

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