Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues:
Direct premiums	$ 2,562,109	$ 2,444,268	$ 2,345,444
Ceded premiums	(1,600,771)	(1,600,559)	(1,595,220)
Net premiums	961,338	843,709	750,224
Commissions and fees	591,317	541,686	537,146
Investment income net of investment expenses	105,882	97,905	89,557
Interest expense on surplus note	(26,865)	(18,880)	(13,048)
Net investment income	79,017	79,025	76,509
Realized investment gains (losses), including other-than- temporary impairment losses	1,339	4,088	(1,738)
Other, net	56,091	50,576	42,058
Total revenues	1,689,102	1,519,084	1,404,199
Benefits and expenses:
Benefits and claims	416,019	367,655	339,315
Amortization of deferred policy acquisition costs	209,399	180,582	157,727
Sales commissions	297,988	272,815	274,893
Insurance expenses	147,280	132,348	123,030
Insurance commissions	21,108	17,783	16,340
Interest expense	28,488	28,691	33,507
Other operating expenses	189,300	181,615	168,406
Total benefits and expenses	1,309,582	1,181,489	1,113,218
Income before income taxes	379,520	337,595	290,981
Income taxes	29,265	118,181	101,110
Net income	$ 350,255	$ 219,414	$ 189,871
Earnings per share:
Basic earnings per share	$ 7.63	$ 4.59	$ 3.70
Diluted earnings per share	$ 7.61	$ 4.59	$ 3.70
Weighted-average shares used in computing earnings per share:
Basic	45,598	47,411	50,881
Diluted	45,689	47,453	50,913
Supplemental disclosures:
Total impairment losses	$ (1,700)	$ (3,420)	$ (6,893)
Impairment losses recognized in other comprehensive income before income taxes	147
Net impairment losses recognized in earnings	(1,553)	(3,420)	(6,893)
Other net realized investment gains	2,892	7,508	5,155
Realized investment gains (losses), including other-than- temporary impairment losses	$ 1,339	$ 4,088	$ (1,738)
Dividends declared per share	$ 0.78	$ 0.70	$ 0.64