Investments (Schedule of Available-for-sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Amortized Cost	$ 1,294,015	$ 1,623,319
Gross Unrealized Gains	602	823
Gross Unrealized Losses	(10,696)	(7,298)
Fair Value	1,283,921	1,616,844
Agency securities [Member]
Investment [Line Items]
Amortized Cost	378,906	441,315
Gross Unrealized Gains	457	509
Gross Unrealized Losses	(3,498)	(2,760)
Fair Value	375,865	439,064
Corporate securities [Member]
Investment [Line Items]
Amortized Cost	638,946	810,444
Gross Unrealized Gains	127	268
Gross Unrealized Losses	(5,689)	(3,020)
Fair Value	633,384	807,692
Municipal securities [Member]
Investment [Line Items]
Amortized Cost	2,500	3,965
Gross Unrealized Gains	5	2
Gross Unrealized Losses	0	(2)
Fair Value	2,505	3,965
Government securities [Member]
Investment [Line Items]
Amortized Cost	273,663	367,595
Gross Unrealized Gains	13	44
Gross Unrealized Losses	(1,509)	(1,516)
Fair Value	$ 272,167	$ 366,123

Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

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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)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
-Paragraph 2
-Subparagraph (c)
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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