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, 2018		$ 66	$ 475,109	$ 1,216,582	$ 84,756	$ (485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised		0
Conversion of common stock units		0
Conversion of restricted stock units		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, 2020, $0.30 per share; 2019, $0.2875 per share				(12,127)
Cumulative effect of change in accounting principle				0
Change in net unrealized investment gains (losses) on fixed maturity securities	113,898				113,898
Change in net funded status of benefit plans	0				0
Acquisition of shares						0
Ending balance at Mar. 31, 2019	1,423,714	66	474,336	1,236,621	198,654	(485,963)
Beginning balance at Dec. 31, 2019	1,567,285	66	480,962	1,352,539	219,681	(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised		0
Conversion of common stock units		0
Conversion of restricted stock units		0
Options exercised and conversion of common stock units and restricted stock units			(178)
Share-based compensation expense			1,133
Net income	18,472			18,472
Dividends, 2020, $0.30 per share; 2019, $0.2875 per share				(12,669)
Cumulative effect of change in accounting principle				(509)
Change in net unrealized investment gains (losses) on fixed maturity securities	(93,769)				(93,769)
Change in net funded status of benefit plans	0				0
Acquisition of shares						(2,145)
Ending balance at Mar. 31, 2020	$ 1,477,620	$ 66	$ 481,917	$ 1,357,833	$ 125,912	$ (488,108)