CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income, net of taxes	Treasury stock, at cost
Beginning balance at Dec. 31, 2015		$ 65	$ 442,648	$ 1,116,277	$ 163,373	$ (457,702)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2017, 127,774 shares; 2016, 94,225 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 8,538 shares		0
Conversion of restricted stock units, 2017, 280,341 shares; 2016, 171,042 shares		0
Options exercised and conversion of common stock units and restricted stock units			939
Share-based compensation expense			3,917
Net income	$ 37,019			37,019
Cash dividends, 2017, $0.55 per share; 2016, $0.53 per share				(22,174)
Change in net unrealized investment gains and losses on fixed maturity and equity securities	154,486				154,486
Change in net funded status of benefit plans	0				0
Acquisition of shares, 2017, 0 shares; 2016, 701,410 shares						(21,513)
Ending balance at Jun. 30, 2016	1,417,335	65	447,504	1,131,122	317,859	(479,215)
Beginning balance at Mar. 31, 2016					232,863
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	11,866
Change in net unrealized investment gains and losses on fixed maturity and equity securities	84,996
Change in net funded status of benefit plans	0
Ending balance at Jun. 30, 2016	1,417,335	65	447,504	1,131,122	317,859	(479,215)
Beginning 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, 127,774 shares; 2016, 94,225 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 8,538 shares		0
Conversion of restricted stock units, 2017, 280,341 shares; 2016, 171,042 shares		0
Options exercised and conversion of common stock units and restricted stock units			1,909
Share-based compensation expense			3,929
Net income	17,579			17,579
Cash dividends, 2017, $0.55 per share; 2016, $0.53 per share				(23,041)
Change in net unrealized investment gains and losses on fixed maturity and equity securities	67,772				67,772
Change in net funded status of benefit plans	0				0
Acquisition of shares, 2017, 0 shares; 2016, 701,410 shares						0
Ending balance at Jun. 30, 2017	1,362,130	65	459,317	1,150,270	231,693	(479,215)
Beginning balance at Mar. 31, 2017					186,454
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	2,261
Change in net unrealized investment gains and losses on fixed maturity and equity securities	45,239
Change in net funded status of benefit plans	0
Ending balance at Jun. 30, 2017	$ 1,362,130	$ 65	$ 459,317	$ 1,150,270	$ 231,693	$ (479,215)