Consolidated Statement of Stockholders' Equity (USD $) In Thousands, unless otherwise specified		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, 2011		$ 696,141	$ 25	$ 213,045	$ 400,485	$ 82,586
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Compensation expense and related tax benefit for stock-based award grants				1,686
Shares repurchased			0	(7,301)
Shares issued for stock-based awards			0	1,106
Net income		40,212			40,212
Dividends on common stock ($0.78 per share in 2014; $0.69 per share in 2013; $0.60 per share in 2012)					(15,269)
Change in net unrealized investment appreciation	[1]					19,720
Change in liability for underfunded employee benefit plans	[2]					(7,118)
All other changes in stockholders' equity accounts		(7,176)
Balance, end 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 ($0.78 per share in 2014; $0.69 per share in 2013; $0.60 per share in 2012)					(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 ($0.78 per share in 2014; $0.69 per share in 2013; $0.60 per share in 2012)					(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

[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.