Consolidated Statement of Stockholders' Equity (Unaudited) - USD ($) $ in Thousands		Total		Cumulative effect of change in accounting principle	Common Stock	Additional paid-in capital	Retaining Earnings	Retaining Earnings Cumulative effect of change in accounting principle	Accumulated other comprehensive income
Balance, beginning (in shares) at Dec. 31, 2018					25,097,408
Balance, beginning at Dec. 31, 2018		$ 888,375		$ (513)	$ 25	$ 203,350	$ 715,472	$ (513)	$ (30,472)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		44,521					44,521
Stock based compensation (in shares)					70,414
Stock based compensation		3,438				3,438
Dividends on common stock		(7,797)					(7,797)
Change in net unrealized investment appreciation	[1]	26,279							26,279
Change in liability for underfunded employee benefit plans	[2]	888							888
Balance, ending (in shares) at Mar. 31, 2019					25,167,822
Balance, ending at Mar. 31, 2019		955,191			$ 25	206,788	751,683		(3,305)
Balance, beginning (in shares) at Dec. 31, 2018					25,097,408
Balance, beginning at Dec. 31, 2018		888,375		(513)	$ 25	203,350	715,472	(513)	(30,472)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		37,983
Balance, ending (in shares) at Sep. 30, 2019					25,081,726
Balance, ending at Sep. 30, 2019		964,168			$ 25	202,504	728,536		33,103
Balance, beginning (in shares) at Mar. 31, 2019					25,167,822
Balance, beginning at Mar. 31, 2019		955,191			$ 25	206,788	751,683		(3,305)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		(4,196)					(4,196)
Shares repurchased (in shares)					(1,507)
Shares repurchased		(69)				(69)
Stock based compensation (in shares)					78,885
Stock based compensation		2,252				2,252
Dividends on common stock		(8,325)					(8,325)
Change in net unrealized investment appreciation	[1]	22,562							22,562
Change in liability for underfunded employee benefit plans	[2]	888							888
Balance, ending (in shares) at Jun. 30, 2019					25,245,200
Balance, ending at Jun. 30, 2019		968,303			$ 25	208,971	739,162		20,145
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		(2,342)					(2,342)
Shares repurchased (in shares)					(177,249)
Shares repurchased		(8,058)				(8,058)
Stock based compensation (in shares)					13,775
Stock based compensation		1,591				1,591
Dividends on common stock		(8,284)					(8,284)
Change in net unrealized investment appreciation	[1]	12,070							12,070
Change in liability for underfunded employee benefit plans	[2]	888							888
Balance, ending (in shares) at Sep. 30, 2019					25,081,726
Balance, ending at Sep. 30, 2019		$ 964,168			$ 25	202,504	728,536		33,103
Balance, beginning (in shares) at Dec. 31, 2019		25,015,963			25,015,963
Balance, beginning at Dec. 31, 2019		$ 910,472		(30)	$ 25	200,179	697,116	(30)	13,152
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		(72,534)					(72,534)
Shares repurchased (in shares)					(70,467)
Shares repurchased		(2,741)				(2,741)
Stock based compensation (in shares)					70,597
Stock based compensation		879				879
Dividends on common stock		(8,249)					(8,249)
Change in net unrealized investment appreciation	[1]	4,500							4,500
Change in liability for underfunded employee benefit plans	[2]	847							847
Balance, ending (in shares) at Mar. 31, 2020					25,016,093
Balance, ending at Mar. 31, 2020		$ 833,144			$ 25	198,317	616,303		18,499
Balance, beginning (in shares) at Dec. 31, 2019		25,015,963			25,015,963
Balance, beginning at Dec. 31, 2019		$ 910,472		$ (30)	$ 25	200,179	697,116	$ (30)	13,152
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		$ (103,815)
Balance, ending (in shares) at Sep. 30, 2020		25,031,234			25,031,234
Balance, ending at Sep. 30, 2020		$ 820,282			$ 25	200,849	568,501		50,907
Balance, beginning (in shares) at Mar. 31, 2020					25,016,093
Balance, beginning at Mar. 31, 2020		833,144			$ 25	198,317	616,303		18,499
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		5,960					5,960
Stock based compensation (in shares)					15,141
Stock based compensation		1,479				1,479
Dividends on common stock		(8,267)					(8,267)
Change in net unrealized investment appreciation	[1]	27,504							27,504
Change in liability for underfunded employee benefit plans	[2]	847							847
Balance, ending (in shares) at Jun. 30, 2020					25,031,234
Balance, ending at Jun. 30, 2020		860,667			$ 25	199,796	613,996		46,850
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		(37,241)					(37,241)
Stock based compensation		1,053				1,053
Dividends on common stock		(8,254)					(8,254)
Change in net unrealized investment appreciation		3,209	[1]						3,209
Change in liability for underfunded employee benefit plans		$ 848	[2]						848
Balance, ending (in shares) at Sep. 30, 2020		25,031,234			25,031,234
Balance, ending at Sep. 30, 2020		$ 820,282			$ 25	$ 200,849	$ 568,501		$ 50,907

[1] The change in net unrealized appreciation is net of reclassification adjustments and income taxes.

[2] The change in liability for underfunded employee benefit plans is net of reclassification adjustments and income taxes.