CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY (UNAUDITED) - USD ($) $ in Thousands	Total	Common stock	Additional paid-in capital	Retained earnings	Accumulated other comprehensive income (loss), net of tax:	Treasury stock, at cost
Beginning balance at Dec. 31, 2017		$ 65	$ 464,246	$ 1,231,177	$ 286,960	$ (480,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2019, 26,094 shares; 2018, 56,614 shares		0
Conversion of common stock units, 2019, 7,585 shares; 2018, 11,804 shares		0
Conversion of restricted stock units, 2019, 146,441 shares; 2018, 105,116 shares		1
Options exercised and conversion of common stock units and restricted stock units			(125)
Share-based compensation expense			2,156
Net income	$ 20,155			20,155
Dividends, 2019, $0.2875 per share; 2018, $0.2850 per share				(11,979)
Change in net unrealized investment gains on securities	(107,096)				(107,096)
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, 2019, 0 shares; 2018, 161 shares						(6)
Ending balance at Mar. 31, 2018	1,404,679	66	466,277	1,254,394	164,823	(480,881)
Beginning balance at Dec. 31, 2018	1,290,550	66	475,109	1,216,582	84,756	(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised, 2019, 26,094 shares; 2018, 56,614 shares		0
Conversion of common stock units, 2019, 7,585 shares; 2018, 11,804 shares		0
Conversion of restricted stock units, 2019, 146,441 shares; 2018, 105,116 shares		0
Options exercised and conversion of common stock units and restricted stock units			(2,104)
Share-based compensation expense			1,331
Net income	32,166			32,166
Dividends, 2019, $0.2875 per share; 2018, $0.2850 per share				(12,127)
Change in net unrealized investment gains on securities	113,898				113,898
Change in net funded status of benefit plans	0				0
Cumulative effect of change in accounting principle	0			0	0
Acquisition of shares, 2019, 0 shares; 2018, 161 shares						0
Ending balance at Mar. 31, 2019	$ 1,423,714	$ 66	$ 474,336	$ 1,236,621	$ 198,654	$ (485,963)