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 tax:	Treasury stock, at cost
Beginning balance at Dec. 31, 2016		$ 65	$ 453,479	$ 1,155,732	$ 163,921	$ (479,215)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2018, 107,148 shares; 2017, 127,774 shares		0
Conversion of common stock units, 2018, 30,368 shares; 2017, 15,981 shares		0
Conversion of restricted stock units, 2018, 163,835 shares; 2017, 280,341 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
Dividends, 2018, $0.57 per share; 2017, $0.55 per share				(23,041)
Change in net unrealized investment gains on securities	67,772				67,772
Change in net funded status of benefit plans	0				0
Cumulative effect of change in accounting principle	0			0	0
Acquisition of shares, 2018, 2,161 shares; 2017, 0 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 on securities	45,239
Change in net funded status of benefit plans	0
Cumulative effect of change in accounting principle	0
Ending balance at Jun. 30, 2017	1,362,130	65	459,317	1,150,270	231,693	(479,215)
Beginning balance at Dec. 31, 2017	1,501,573	65	464,246	1,231,177	286,960	(480,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2018, 107,148 shares; 2017, 127,774 shares		0
Conversion of common stock units, 2018, 30,368 shares; 2017, 15,981 shares		0
Conversion of restricted stock units, 2018, 163,835 shares; 2017, 280,341 shares		1
Options exercised and conversion of common stock units and restricted stock units			2,259
Share-based compensation expense			4,147
Net income	26,072			26,072
Dividends, 2018, $0.57 per share; 2017, $0.55 per share				(23,985)
Change in net unrealized investment gains on securities	(159,540)				(159,540)
Change in net funded status of benefit plans	0				0
Cumulative effect of change in accounting principle	(15,041)			15,041	(15,041)
Acquisition of shares, 2018, 2,161 shares; 2017, 0 shares						(86)
Ending balance at Jun. 30, 2018	1,350,441	66	470,652	1,248,305	112,379	(480,961)
Beginning balance at Mar. 31, 2018					164,823
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	5,917
Change in net unrealized investment gains on securities	(52,444)
Change in net funded status of benefit plans	0
Cumulative effect of change in accounting principle	0
Ending balance at Jun. 30, 2018	$ 1,350,441	$ 66	$ 470,652	$ 1,248,305	$ 112,379	$ (480,961)