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, 56,614 shares; 2017, 33,764 shares		0
Conversion of common stock units, 2018, 11,804 shares; 2017, 15,981 shares		0
Conversion of restricted stock units, 2018, 105,116 shares; 2017, 247,620 shares		0
Options exercised and conversion of common stock units and restricted stock units			(750)
Share-based compensation expense			2,253
Net income	$ 15,318			15,318
Dividends, 2018, $0.285 per share; 2017, $0.275 per share				(11,518)
Change in net unrealized investment gains (losses) on securities	22,533				22,533
Change in net funded status of benefit plans	0				0
Cumulative effect of change in accounting principle	0			0	0
Acquisition of shares, 2018, 161 shares; 2017, 0 shares						0
Ending balance at Mar. 31, 2017	1,321,818	65	454,982	1,159,532	186,454	(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, 56,614 shares; 2017, 33,764 shares		0
Conversion of common stock units, 2018, 11,804 shares; 2017, 15,981 shares		0
Conversion of restricted stock units, 2018, 105,116 shares; 2017, 247,620 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, 2018, $0.285 per share; 2017, $0.275 per share				(11,979)
Change in net unrealized investment gains (losses) 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, 2018, 161 shares; 2017, 0 shares						(6)
Ending balance at Mar. 31, 2018	$ 1,404,679	$ 66	$ 466,277	$ 1,254,394	$ 164,823	$ (480,881)