Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Cash flows from operating activities:
Net income	$ 20,813	$ 18,735
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	6,388	5,563
Deferred income taxes	(608)	1,097
Stock-based compensation	1,215	1,106
Net change in accrued interest, premiums, and discounts on investments	424	556
Other	26
Net Cash Provided By Used In Operating Activities Before Changes In Operating Capital, Total	28,258	27,057
Changes in operating assets and liabilities:
Accounts receivable	(3,129)	(3,356)
Inventories	(2,709)	70
Prepaid expenses	1,518	823
Other non-current assets	(17)	(96)
Accounts payable and accrued liabilities	2,525	(543)
Accrued income and other taxes	1,172	135
Other non-current liabilities	(198)	(723)
Net Cash Provided by (Used in) Operating Activities, Total	27,420	23,367
Cash flows from investing activities:
Property, plant and equipment additions	(4,713)	(7,978)
Purchase of investments		(21,545)
Proceeds from maturities of investments	7,639	11,750
Net Cash Provided by (Used in) Investing Activities, Total	2,926	(17,773)
Cash flows from financing activities:
Shares tendered for employees' withholding taxes on stock-based compensation		(1,065)
Issuance of treasury stock		153
Tax benefit related to stock-based compensation	6	929
Purchase of treasury stock	(4,086)	(4,756)
Dividends paid	(3,541)	(3,104)
Net Cash Provided by (Used in) Financing Activities, Total	(7,621)	(7,843)
Net change in cash and cash equivalents	22,725	(2,249)
Cash and cash equivalents at beginning of period	7,999	24,590
Cash and cash equivalents at end of period	30,724	22,341
Cash paid for:
Income taxes	$ 7,927	$ 6,471