Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Condensed Consolidated Statements of Cash Flows
Net Cash Provided by Operating Activities	$ 145,933	$ 123,000
Net Cash Provided by (Used in) Investing Activities [Abstract]
Investments purchased	(335,361)	(434,505)
Investments sold	131,605	243,506
Investments called or matured	103,193	102,752
Net change in short-term investments	(6,910)	(1,476)
Net property and equipment purchased	(7,262)	(11,852)
Net proceeds from sale of agency	408
Acquisition of agency		(850)
Net Cash Used in Investing Activities, Total	(114,327)	(102,425)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Cash dividends paid	(27,288)	(25,811)
Proceeds from stock option exercises	4,290	(357)
Excess tax benefit from exercise of stock options		8,482
Net Cash Used in Financing Activities, Total	(22,998)	(17,686)
Net increase in cash	8,608	2,889
Cash at the beginning of the period	18,269	11,081
Cash at the end of the period	$ 26,877	$ 13,970