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Stockholders Equity - Additional Information (Detail) - USD ($) shares in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Stockholders Equity Note [Line Items]
Proceeds from common stock sold	$ 7,199,000	$ 75,527,000
Cantor | Sales Agreement
Stockholders Equity Note [Line Items]
Maximum common stock offering price	$ 70,000,000
Percentage of proceeds payable as compensation to underwriter	2.00%
Common stock, number of shares issued	1.2
Proceeds from common stock sold	$ 7,100,000
Common stock registered for sale	$ 15,300,000

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Financing Activities
-URI http://asc.fasb.org/extlink&oid=6513228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3255-108585

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-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644

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-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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-Name Regulation S-X (SX)
-Number 210
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