Consolidated Statements Of Operations - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Net premiums earned	$ 925,384	$ 915,012	$ 888,776	$ 870,245	$ 868,233	$ 858,135	$ 833,959	$ 808,084	$ 3,599,418	$ 3,368,411	$ 3,195,437
Net investment income									141,263	135,838	124,930
Net realized investment gains (losses)									222,793	(133,520)	83,650
Other									9,044	9,275	11,945
Total revenues									3,972,518	3,380,004	3,415,962
Expenses:
Losses and loss adjustment expenses									2,706,024	2,576,789	2,444,884
Policy acquisition costs									602,085	572,164	555,350
Other operating expenses									269,305	244,630	233,475
Interest									17,035	17,036	15,168
Total expenses									3,594,449	3,410,619	3,248,877
Income (loss) before income taxes	28,465	80,840	101,595	167,169	(114,448)	70,286	73,246	(59,699)	378,069	(30,615)	167,085
Income tax expense (benefit)									57,982	(24,887)	22,208
Net income (loss)	$ 31,688	$ 69,282	$ 83,250	$ 135,867	$ (81,879)	$ 58,578	$ 60,180	$ (42,607)	$ 320,087	$ (5,728)	$ 144,877
Net income (loss) per share:
Basic (in dollars per share)	$ 0.57	$ 1.25	$ 1.50	$ 2.46	$ (1.48)	$ 1.06	$ 1.09	$ (0.77)	$ 5.78	$ (0.10)	$ 2.62
Diluted (in dollars per share)	$ 0.57	$ 1.25	$ 1.50	$ 2.45	$ (1.48)	$ 1.06	$ 1.09	$ (0.77)	$ 5.78	$ (0.10)	$ 2.62