CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Cumulative effect of change in accounting principle	Common stock	Additional paid-in capital	Retained earnings	Retained earnings Cumulative effect of change in accounting principle	Accumulated other comprehensive income (loss), net of tax:	Accumulated other comprehensive income (loss), net of tax: Cumulative effect of change in accounting principle	Treasury stock, at cost
Beginning balance at Dec. 31, 2017		$ (15,041)	$ 65	$ 464,246	$ 1,231,177	$ 15,041	$ 286,960	$ (15,041)	$ (480,875)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of restricted common stock units			1
Options exercised and conversion of common stock units and restricted stock units				3,008
Share-based compensation expense				7,855
Net income	$ 18,343				18,343
Dividends, 2020, $1.20 per share; 2019, $1.15 per share; 2018, $1.14 per share					(47,979)
Change in net unrealized investment gains (losses) on fixed maturity securities	(188,195)						(188,195)
Change in net funded status of benefit plans	1,032						1,032
Acquisition of shares									(5,088)
Ending balance at Dec. 31, 2018	1,290,550	0	66	475,109	1,216,582		84,756		(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units				(555)
Share-based compensation expense				6,408
Net income	184,443				184,443
Dividends, 2020, $1.20 per share; 2019, $1.15 per share; 2018, $1.14 per share					(48,486)
Change in net unrealized investment gains (losses) on fixed maturity securities	133,507						133,507
Change in net funded status of benefit plans	1,418						1,418
Ending balance at Dec. 31, 2019	1,567,285	$ 0	66	480,962	1,352,539	$ (509)	219,681		(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units				1,489
Share-based compensation expense				5,916
Net income	$ 133,315				133,315
Dividends, 2020, $1.20 per share; 2019, $1.15 per share; 2018, $1.14 per share					(50,711)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Change in net unrealized investment gains (losses) on fixed maturity securities	$ 135,837						135,837
Change in net funded status of benefit plans	(398)						(398)
Acquisition of shares									(2,145)
Ending balance at Dec. 31, 2020	$ 1,790,079		$ 66	$ 488,367	$ 1,434,634		$ 355,120		$ (488,108)