Fair Value Measurements - Schedule of Fair Value of Long-term Borrowings (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Long-term Borrowings:
Carrying amount of senior notes	$ 303,416	$ 221,082
5.0% Exchangeable Senior Notes due March 2021 [Member] \| Exchangeable Senior Notes [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Long-term Borrowings:
Carrying amount of senior notes	81,576
Estimated fair value of senior notes	127,617
6.625% Senior Notes Due 2021 [Member] \| Senior Notes [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Long-term Borrowings:
Carrying amount of senior notes	221,840	221,082
Estimated fair value of senior notes	$ 224,719	$ 193,500

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

Reference 1: 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_LongTermDebtNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant