General - Investments (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Short-term and long-term investments
Other-than-temporary unrealized losses	$ 0		$ 0
Realized gains or losses	0	$ 0
Amortized Cost	323,719		305,540
Gross Unrealized Gains	1		3
Gross Unrealized Losses	(243)		(290)
Estimated Fair Value	323,477		305,253
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	8,689		5,832
Gross Unrealized Losses	(21)		(15)
Estimated Fair Value	8,668		5,817
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	13,279		25,779
Gross Unrealized Gains			2
Gross Unrealized Losses	(15)		(14)
Estimated Fair Value	13,264		25,767
Corporate debt securities
Short-term and long-term investments
Amortized Cost	295,384		272,479
Gross Unrealized Gains	1		1
Gross Unrealized Losses	(204)		(261)
Estimated Fair Value	295,181		272,219
Certificates of deposit
Short-term and long-term investments
Amortized Cost	1,350		1,450
Estimated Fair Value	1,350		$ 1,450
Taxable municipal bonds
Short-term and long-term investments
Amortized Cost	5,017
Gross Unrealized Losses	(3)
Estimated Fair Value	$ 5,014

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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