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			(1,761)
Share-based compensation expense			3,005
Net income	$ 125,988			125,988
Dividends, 2020, $0.30, $0.60 per share; 2019, $0.2875, $0.5750 per share				(24,241)
Change in net unrealized investment gains (losses) on fixed maturity securities					106,136
Ending balance at Jun. 30, 2019	1,499,677	66	476,353	1,318,329	190,892	(485,963)
Beginning balance at Mar. 31, 2019		66	474,336	1,236,621	198,654	(485,963)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			344
Share-based compensation expense			1,673
Net income	93,822			93,822
Dividends, 2020, $0.30, $0.60 per share; 2019, $0.2875, $0.5750 per share				(12,114)
Change in net unrealized investment gains (losses) on fixed maturity securities					(7,762)
Ending balance at Jun. 30, 2019	1,499,677	66	476,353	1,318,329	190,892	(485,963)
Beginning balance at Dec. 31, 2019	1,567,285	66	480,962	1,352,539	219,681	(485,963)	$ (509)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			268
Share-based compensation expense			2,524
Net income	49,050			49,050
Dividends, 2020, $0.30, $0.60 per share; 2019, $0.2875, $0.5750 per share				(25,343)
Change in net unrealized investment gains (losses) on fixed maturity securities					48,681
Acquisition of shares						(2,145)
Ending balance at Jun. 30, 2020	1,639,811	66	483,754	1,375,737	268,362	(488,108)
Beginning balance at Mar. 31, 2020		66	481,917	1,357,833	125,912	(488,108)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Options exercised and conversion of common stock units and restricted stock units			447
Share-based compensation expense			1,390
Net income	30,578			30,578
Dividends, 2020, $0.30, $0.60 per share; 2019, $0.2875, $0.5750 per share				(12,674)
Change in net unrealized investment gains (losses) on fixed maturity securities					142,450
Ending balance at Jun. 30, 2020	$ 1,639,811	$ 66	$ 483,754	$ 1,375,737	$ 268,362	$ (488,108)