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, 2013		$ 782,833	$ 25	$ 211,574	$ 484,084	$ 87,150
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares repurchased			0	(12,942)
Shares issued for stock-based awards			0	2,260
Compensation expense and related tax benefit for stock-based award grants				1,784
Net income		59,137			59,137
Dividends on common stock ($0.97 per share in 2016; $0.86 per share in 2015; $0.78 per share in 2014)					(19,680)
Change in net unrealized investment appreciation	[1]					33,022
Change in liability for underfunded employee benefit plans	[2]					(28,999)
All other changes in stockholders' equity accounts		(24,555)
Balance, end 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
Dividends on common stock ($0.97 per share in 2016; $0.86 per share in 2015; $0.78 per share in 2014)					(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
Dividends on common stock ($0.97 per share in 2016; $0.86 per share in 2015; $0.78 per share in 2014)					(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

[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.