Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands		Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income, net of tax
Balance, beginning of year at Dec. 31, 2014		$ 817,415	$ 25	$ 202,676	$ 523,541	$ 91,173
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			0	(2,423)
Shares issued for stock-based awards			0	5,496
Compensation expense and related tax benefit for stock-based award grants				1,677
Net income		89,126			89,126
Accumulated effect of change in enacted tax rate					0	0
Dividends on common stock ($1.09 per share in 2017; $0.97 per share in 2016; $0.86 per share in 2015)					(21,658)
Change in net unrealized investment appreciation	[1]					(21,254)
Change in liability for underfunded employee benefit plans	[2]					10,518
All other changes in stockholders' equity accounts		(27,644)
Balance, end of year at Dec. 31, 2015		878,897	25	207,426	591,009	80,437
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			0	(3,746)
Shares issued for stock-based awards			0	9,922
Compensation expense and related tax benefit for stock-based award grants				2,880
Net income		49,904			49,904
Accumulated effect of change in enacted tax rate					0	0
Dividends on common stock ($1.09 per share in 2017; $0.97 per share in 2016; $0.86 per share in 2015)					(24,591)
Change in net unrealized investment appreciation	[1]					5,523
Change in liability for underfunded employee benefit plans	[2]					23,095
All other changes in stockholders' equity accounts		13,083
Balance, end of year at Dec. 31, 2016		941,884	25	216,482	616,322	109,055
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			0	(29,784)
Shares issued for stock-based awards			0	4,828
Compensation expense and related tax benefit for stock-based award grants				4,808
Net income		51,023			51,023
Accumulated effect of change in enacted tax rate		31,308			(31,308)	31,308
Dividends on common stock ($1.09 per share in 2017; $0.97 per share in 2016; $0.86 per share in 2015)					(27,337)
Change in net unrealized investment appreciation	[1]					44,315
Change in liability for underfunded employee benefit plans	[2]					(16,364)
All other changes in stockholders' equity accounts		(19,534)
Balance, end of year at Dec. 31, 2017		$ 973,373	$ 25	$ 196,334	$ 608,700	$ 168,314

[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 income taxes.