Investments, Gains Losses on Investments, Including Equity Method Earnings from Limited Partnerships (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains										$ 10,807		$ 19,616		$ 40,370
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses										(14,367)		(12,250)		(13,872)
Other-than-temporary impairments										(19)		(149)		(5,743)
Change in value of limited partnership investments										(9,343)		12,469		2,473
Losses on equity securities [Abstract]
Realized losses on equity securities sold during the period										(3,072)	[1]	0	[1]	0
Unrealized losses on equity securities held at the end of the period										(9,697)		0		0
Realized and unrealized losses on equity securities held at the end of the period										(12,769)		0		0
Net realized and unrealized gains (losses) on investments		$ (20,096)	$ 2,373	$ (3,435)	$ (4,533)	$ 4,152	$ 5,944	$ 3,296	$ 6,294	(25,691)		19,686		23,228
Proceeds from sales of equity securities										149,195		69,756		88,773
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										10,807		9,135		11,628
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses										(14,367)		(9,882)		(10,940)
Equity Securities [Member]
Gross gains on available-for-sale investments sold during the period [Abstract]
Total gains	[2]									0		10,481		28,742
Gross losses on available-for-sale investments sold during the period [Abstract]
Total losses	[2]									$ 0		$ (2,368)		$ (2,932)

[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.

[2] Effective January 1, 2018, the Company adopted ASU 2016-01 and equity securities are no longer classified as available-for-sale. Prior periods have not been restated to conform to the current presentation. See Note A - Summary of Significant Accounting Policies - Recently Adopted Accounting Pronouncements for further discussion.