Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Condensed Consolidated Statements of Cash Flows
Net Cash Provided by Operating Activities	$ 100,000	$ 74,570
Net Cash Provided by (Used in) Investing Activities [Abstract]
Investments purchased	(431,061)	(207,963)
Investments sold	282,276	110,679
Investments called or matured	67,493	56,501
Net change in short-term investments	9,980	(10,995)
Net property and equipment purchased	(3,638)	(5,372)
Proceeds from (used in) other investing activities	62	(92)
Net Cash Used in Investing Activities, Total	(74,888)	(57,242)
Net Cash Provided by (Used in) Financing Activities [Abstract]
Cash dividends paid	(19,043)	(18,036)
Proceeds from stock option exercises	3,762	3,794
Net Cash Used in Financing Activities, Total	(15,281)	(14,242)
Net increase in cash	9,831	3,086
Cash at the beginning of the period	24,271	18,269
Cash at the end of the period	$ 34,102	$ 21,355