Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common shares:	Treasury shares:	Additional paid-in capital:	Accumulated other comprehensive income:	Retained earnings:
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption					$ 0
Balance at beginning of year (shares) at Dec. 31, 2015		48,100,000	6,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares (shares)		500,000
Balance at end of year (shares) at Dec. 31, 2016		48,600,000	6,800,000
Balance at beginning of year at Dec. 31, 2015		$ 120,400	$ (116,300)	$ 153,500	700	$ 685,300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		1,200		6,800
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				(3,000)
Shares acquired on stock award exercises and vested restricted shares			(200)
Stock awards granted				2,600
Change in unrealized holding gains (losses) on investments, net of tax	$ (5,700)				(5,700)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	2,600				2,600
Net (loss) income	19,200					19,200
Cash dividends paid (affiliates $10.4, $10.4 and $10.4, respectively)		(16,600)
Balance at end of year at Dec. 31, 2016	850,500	$ 121,600	$ (116,500)	159,900	(2,400)	687,900
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption					0	0
Issuance of shares (shares)		600,000
Balance at end of year (shares) at Dec. 31, 2017		49,200,000	6,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 1,400		8,800
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0
Shares acquired on stock award exercises and vested restricted shares			$ (300)
Stock awards granted				3,100
Change in unrealized holding gains (losses) on investments, net of tax	3,200				3,200
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	3,000				3,000
Net (loss) income	(17,800)					(17,800)
Cash dividends paid (affiliates $10.4, $10.4 and $10.4, respectively)		(16,900)
Balance at end of year at Dec. 31, 2017	835,000	$ 123,000	$ (116,800)	171,800	3,800	653,200
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption						0
Issuance of shares (shares)		800,000
Balance at end of year (shares) at Dec. 31, 2018		50,000,000.0	6,800,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of shares		$ 2,000		13,300
Adjustments to Additional Paid in Capital, Income Tax Benefit from Share-based Compensation				0
Shares acquired on stock award exercises and vested restricted shares			$ (200)
Stock awards granted				9,100
Change in unrealized holding gains (losses) on investments, net of tax	(38,300)				(38,300)
Change in unrecognized benefit plan obligations, net of tax and reclassification adjustments	2,000				2,000
Net (loss) income	12,800					12,800
Cash dividends paid (affiliates $10.4, $10.4 and $10.4, respectively)		(17,200)
Balance at end of year at Dec. 31, 2018	$ 818,500	$ 125,000	$ (117,000)	$ 194,200	$ (96,400)	$ 712,700