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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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

Summary of Significant Accounting Policies - Convertible Senior Notes (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Nov. 30, 2013
2022 Notes
Debt Instrument [Line Items]
Notes, par value	$ 782,500,000	$ 782,500,000	$ 782,500,000
Contractual interest rate, notes			0.00%
2018 Notes
Debt Instrument [Line Items]
Notes, par value	$ 0	$ 574,994,000		$ 575,000,000.0
Contractual interest rate, notes				0.00%