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, 156,211 shares; 2016, 114,507 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 15,629 shares		0
Conversion of restricted stock units, 2017, 293,002 shares; 2016, 188,207 shares		0
Options exercised and conversion of common stock units and restricted stock units			2,045
Share-based compensation expense			6,066
Net income	$ 63,942			63,942
Cash dividends, 2017, $0.825 per share; 2016, $0.795 per share				(33,241)
Change in net unrealized investment gains on fixed maturity and equity securities	162,124				162,124
Change in net funded status of benefit plans	0				0
Acquisition of shares, 2017, 48,440 shares; 2016, 701,410 shares						(21,513)
Ending balance at Sep. 30, 2016	1,444,084	65	450,759	1,146,978	325,497	(479,215)
Beginning balance at Jun. 30, 2016					317,859
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,923
Change in net unrealized investment gains on fixed maturity and equity securities	7,638
Change in net funded status of benefit plans	0
Ending balance at Sep. 30, 2016	1,444,084	65	450,759	1,146,978	325,497	(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, 156,211 shares; 2016, 114,507 shares		0
Conversion of common stock units, 2017, 15,981 shares; 2016, 15,629 shares		0
Conversion of restricted stock units, 2017, 293,002 shares; 2016, 188,207 shares		0
Options exercised and conversion of common stock units and restricted stock units			2,773
Share-based compensation expense			5,816
Net income	44,130			44,130
Cash dividends, 2017, $0.825 per share; 2016, $0.795 per share				(34,580)
Change in net unrealized investment gains on fixed maturity and equity securities	79,980				79,980
Change in net funded status of benefit plans	0				0
Acquisition of shares, 2017, 48,440 shares; 2016, 701,410 shares						(1,660)
Ending balance at Sep. 30, 2017	1,390,441	65	462,068	1,165,282	243,901	(480,875)
Beginning balance at Jun. 30, 2017					231,693
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,551
Change in net unrealized investment gains on fixed maturity and equity securities	12,208
Change in net funded status of benefit plans	0
Ending balance at Sep. 30, 2017	$ 1,390,441	$ 65	$ 462,068	$ 1,165,282	$ 243,901	$ (480,875)