Summary of Significant Accounting Policies - Convertible Notes (Details) - Convertible notes - USD ($)	Dec. 31, 2017	Jun. 17, 2013
The 2018 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued		1.50%
Debt issued, senior convertible notes		$ 253,000,000
The 2021 Notes
Debt Instrument [Line Items]
Interest rate on convertible notes issued	5.75%
Debt issued, senior convertible notes	$ 300,000,000

Face (par) amount of debt instrument at time of issuance.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant