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	Cumulative Effect, Period of Adoption, Adjustment Retained earnings
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 and conversion of common stock units and restricted stock units			(1,314)
Share-based compensation expense			4,855
Net income	$ 151,442			151,442
Dividends, 2020, $0.30, $0.90 per share; 2019, $0.2875, $0.8625 per share				(36,361)
Change in net unrealized investment gains (losses) on fixed maturity securities					169,440
Ending balance at Sep. 30, 2019	1,578,612	66	478,650	1,331,663	254,196	(485,963)
Beginning balance at Jun. 30, 2019		66	476,353	1,318,329	190,892	(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			447
Share-based compensation expense			1,850
Net income	25,454			25,454
Dividends, 2020, $0.30, $0.90 per share; 2019, $0.2875, $0.8625 per share				(12,120)
Change in net unrealized investment gains (losses) on fixed maturity securities					63,304
Ending balance at Sep. 30, 2019	1,578,612	66	478,650	1,331,663	254,196	(485,963)
Beginning balance at Dec. 31, 2019	1,567,285	66	480,962	1,352,539	219,681	(488,108)	$ (509)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			1,587
Share-based compensation expense			4,214
Net income	85,524			85,524
Dividends, 2020, $0.30, $0.90 per share; 2019, $0.2875, $0.8625 per share				(38,027)
Change in net unrealized investment gains (losses) on fixed maturity securities					97,749
Acquisition of shares						(2,145)
Ending balance at Sep. 30, 2020	1,715,678	66	486,763	1,399,527	317,430	(488,108)
Beginning balance at Jun. 30, 2020		66	483,754	1,375,737	268,362	(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			1,319
Share-based compensation expense			1,690
Net income	36,474			36,474
Dividends, 2020, $0.30, $0.90 per share; 2019, $0.2875, $0.8625 per share				(12,684)
Change in net unrealized investment gains (losses) on fixed maturity securities					49,068
Ending balance at Sep. 30, 2020	$ 1,715,678	$ 66	$ 486,763	$ 1,399,527	$ 317,430	$ (488,108)