General - Investments (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Short-term and long-term investments
Other-than-temporary unrealized losses	$ 0		$ 0
Realized gains or losses	0	$ 0
Amortized Cost	351,441		328,364
Gross Unrealized Gains	11		1
Gross Unrealized Losses	(931)		(500)
Estimated Fair Value	350,521		327,865
U.S. Government and agency securities
Short-term and long-term investments
Amortized Cost	33,311		28,313
Gross Unrealized Losses	(141)		(82)
Estimated Fair Value	33,170		28,231
Obligations of government-sponsored enterprises
Short-term and long-term investments
Amortized Cost	22,163		22,139
Gross Unrealized Gains	3
Gross Unrealized Losses	(57)		(51)
Estimated Fair Value	22,109		22,088
Corporate debt securities
Short-term and long-term investments
Amortized Cost	293,205		270,154
Gross Unrealized Gains	8		1
Gross Unrealized Losses	(733)		(367)
Estimated Fair Value	292,480		269,788
Taxable municipal bonds
Short-term and long-term investments
Amortized Cost			5,000
Estimated Fair Value			5,000
Certificates of deposit
Short-term and long-term investments
Amortized Cost	2,762		2,758
Estimated Fair Value	$ 2,762		$ 2,758

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
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-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
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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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