CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of taxes	Treasury stock, at cost
Beginning balance at Dec. 31, 2014		$ 64	$ 422,232	$ 1,065,318	$ 284,601	$ (435,752)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2017, 208,306 shares; 2016, 142,203 shares; 2015, 85,532 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 15,629 shares; 2015, 8,293 shares		0
Conversion of restricted stock units, 2017, 313,292 shares; 2016, 222,297 shares; 2015, 198,681 shares		1
Options exercised and conversion of common stock units and restricted stock units			13,605
Share-based compensation expense			6,811
Net income	$ 93,482			93,482
Cash dividends, 2017, $1.10 per share; 2016, $1.06 per share; 2015, $1.00 per share				(42,523)
Reclassification of deferred taxes				0	0
Change in net unrealized investment gains and losses on fixed maturity and equity securities	(122,387)				(122,387)
Change in net funded status of benefit plans	1,159				1,159
Acquisition of shares, 2017, 48,440 shares; 2016, 701,410 shares; 2015, 663,092 shares						(21,950)
Ending balance at Dec. 31, 2015	1,264,661	65	442,648	1,116,277	163,373	(457,702)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2017, 208,306 shares; 2016, 142,203 shares; 2015, 85,532 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 15,629 shares; 2015, 8,293 shares		0
Conversion of restricted stock units, 2017, 313,292 shares; 2016, 222,297 shares; 2015, 198,681 shares		0
Options exercised and conversion of common stock units and restricted stock units			2,696
Share-based compensation expense			8,135
Net income	83,765			83,765
Cash dividends, 2017, $1.10 per share; 2016, $1.06 per share; 2015, $1.00 per share				(44,310)
Reclassification of deferred taxes				0	0
Change in net unrealized investment gains and losses on fixed maturity and equity securities	571				571
Change in net funded status of benefit plans	(23)				(23)
Acquisition of shares, 2017, 48,440 shares; 2016, 701,410 shares; 2015, 663,092 shares						(21,513)
Ending balance at Dec. 31, 2016	1,293,982	65	453,479	1,155,732	163,921	(479,215)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2017, 208,306 shares; 2016, 142,203 shares; 2015, 85,532 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 15,629 shares; 2015, 8,293 shares		0
Conversion of restricted stock units, 2017, 313,292 shares; 2016, 222,297 shares; 2015, 198,681 shares		0
Options exercised and conversion of common stock units and restricted stock units			2,962
Share-based compensation expense			7,805
Net income	169,459			169,459
Cash dividends, 2017, $1.10 per share; 2016, $1.06 per share; 2015, $1.00 per share				(46,114)
Reclassification of deferred taxes	47,900			(47,900)	47,900
Change in net unrealized investment gains and losses on fixed maturity and equity securities	74,405				74,405
Change in net funded status of benefit plans	734				734
Acquisition of shares, 2017, 48,440 shares; 2016, 701,410 shares; 2015, 663,092 shares						(1,660)
Ending balance at Dec. 31, 2017	$ 1,501,573	$ 65	$ 464,246	$ 1,231,177	$ 286,960	$ (480,875)