Investments, Gains Losses on Investments, Including Equity Method Earnings from Limited Partnerships (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains										$ 11,009	$ 10,807	$ 19,616
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses										(10,492)	(14,367)	(12,250)
Other-than-temporary impairments										(497)	(19)	(149)
Change in value of limited partnership investments										1,644	(9,343)	12,469
Gains (losses) on equity securities [Abstract]
Realized gains (losses) on equity securities sold during the period										1,938	(3,072)	0
Unrealized gains (losses) on equity securities held at the end of the period										9,287	(9,697)	0
Total realized and unrealized gains (losses) on equity securities										11,225	(12,769)	0
Net realized and unrealized gains (losses) on investments		$ 3,848	$ 125	$ 2,889	$ 6,027	$ (20,096)	$ 2,373	$ (3,435)	$ (4,533)	12,889	(25,691)	19,686
Proceeds from sales of equity securities										21,621	149,195	69,756
Realized gains on equity securities											51,900
Fixed Income Securities [Member]
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains										11,009	10,807	9,135
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses										(10,492)	(14,367)	(9,882)
Equity Securities [Member]
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains	[1]									0	0	10,481
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses	[1]									$ 0	$ 0	$ (2,368)

[1] Effective January 1, 2018, the Company adopted Accounting Standards Update ("ASU") 2016-01, Financial Instruments - Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, or ASU 2016-01, and equity securities are no longer classified as available-for-sale. The amendments in ASU 2016-01 changed the accounting for equity securities by requiring changes in fair value to be recognized in income. Previously, the Company's equity securities were classified as available-for-sale and changes in fair value were recognized in accumulated other comprehensive income (loss) as a component of shareholders' equity. Prior periods have not been restated to conform to the current presentation.