Available-for-Sale Securities - Summary of Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 256,290
Aggregate Fair Value	256,278
U.S. Treasury Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	256,290	$ 50,004
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(12)	(7)
Aggregate Fair Value	$ 256,278	$ 49,997

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

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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.

+ References

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=73527792&loc=d3e27161-111563

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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
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-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 45
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-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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Line items represent financial 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.

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