Long-Term Debt and Long-Term Debt Payable to Related Parties - Carrying Amounts and Estimated Fair Values of Senior Notes (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Promissory Notes	$ 412,500	$ 412,500
Fair Value	425,400	429,675
2019 Promissory Notes [Member]
Debt Instrument [Line Items]
Promissory Notes	300,000	300,000
Fair Value	309,750	313,125
2042 Promissory Notes [Member]
Debt Instrument [Line Items]
Promissory Notes	112,500	112,500
Fair Value	$ 115,650	$ 116,550

The amount for notes payable (written promise to pay), payable to related parties, which are due after one year (or one business cycle).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(k)(1))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6457730&loc=d3e39549-107864

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.23)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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