Long-Term Debt - Summary of Principal and Interest Payment Terms of Convertible Senior Notes (Detail)	12 Months Ended
Dec. 31, 2017
3.875% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Convertible senior notes, interest payment terms	Semiannually in arrears: March 15 and September 15
4.25% Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Convertible senior notes, interest payment terms	Semiannually in arrears: March 1 and September 1

Description of the frequency of periodic payments (monthly, quarterly, annual).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentFrequencyOfPeriodicPayment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration