Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands		Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income, Net of Tax [Member]
Balance, beginning of year at Dec. 31, 2012		$ 729,177	$ 25	$ 208,536	$ 425,428	$ 95,188
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Compensation expense and related tax benefit for stock-based award grants				1,289
Shares repurchased			0	(1,644)
Shares issued for stock-based awards			0	3,393
Net income		76,140			76,140
Dividends on common stock ($0.86 per share in 2015; $0.78 per share in 2014; $0.69 per share in 2013)					(17,484)
Change in net unrealized investment appreciation	[1]					(27,495)
Change in liability for underfunded employee benefit plans	[2]					19,457
All other changes in stockholders' equity accounts		(22,484)
Balance, end of year at Dec. 31, 2013		782,833	25	211,574	484,084	87,150
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Compensation expense and related tax benefit for stock-based award grants				1,784
Shares repurchased			0	(12,942)
Shares issued for stock-based awards			0	2,260
Net income		59,137			59,137
Dividends on common stock ($0.86 per share in 2015; $0.78 per share in 2014; $0.69 per share in 2013)					(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]
Compensation expense and related tax benefit for stock-based award grants				1,677
Shares repurchased			0	(2,423)
Shares issued for stock-based awards			0	5,496
Net income		89,126			89,126
Dividends on common stock ($0.86 per share in 2015; $0.78 per share in 2014; $0.69 per share in 2013)					(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

[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.