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, 2015		$ 878,897	$ 25	$ 207,426	$ 591,009	$ 80,437
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			0	(3,746)
Compensation expense and related tax benefit for stock-based award grants				2,880
Shares issued for stock-based awards			0	9,922
Accumulated effect of change in enacted tax rate					0	0
Change in net unrealized investment appreciation					0	5,523	[1]
Net income		49,904			49,904
Dividends on common stock ($4.21 per share in 2018; $1.09 per share in 2017; $0.97 per share in 2016)					(24,591)
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]
Cumulative effect of change in accounting principle						0
Shares repurchased			0	(29,784)
Compensation expense and related tax benefit for stock-based award grants				4,808
Shares issued for stock-based awards			0	4,828
Accumulated effect of change in enacted tax rate		31,308			(31,308)	31,308
Change in net unrealized investment appreciation					0	44,315	[1]
Net income		51,023			51,023
Dividends on common stock ($4.21 per share in 2018; $1.09 per share in 2017; $0.97 per share in 2016)					(27,337)
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle						0
Shares repurchased			0	(5,404)
Compensation expense and related tax benefit for stock-based award grants				5,249
Shares issued for stock-based awards			0	7,171
Accumulated effect of change in enacted tax rate					0	0
Change in net unrealized investment appreciation					(6,714)	(32,944)	[1]
Net income		27,650			27,650
Dividends on common stock ($4.21 per share in 2018; $1.09 per share in 2017; $0.97 per share in 2016)					(105,408)
Change in liability for underfunded employee benefit plans	[2]					25,402
All other changes in stockholders' equity accounts		(112,648)
Balance, end of year at Dec. 31, 2018		$ 888,375	$ 25	$ 203,350	$ 715,472	(30,472)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of change in accounting principle						$ (191,244)

[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.