Fair Value Measurements - Schedule of Fair Value Information for Financial Assets and Liabilities Carried on Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financial Assets:
Limited partnership investments	$ 28,944	$ 23,184
Financial Liabilities:
Total long-term debt	248,544	237,835
Estimate of Fair Value Measurement [Member]
Financial Assets:
Limited partnership investments	28,944	23,184
Financial Liabilities:
Total long-term debt	262,084	240,212
Estimate of Fair Value Measurement [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	89,315	90,827
Estimate of Fair Value Measurement [Member] \| 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	142,133	124,444
Estimate of Fair Value Measurement [Member] \| 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,144	9,227
Estimate of Fair Value Measurement [Member] \| 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,739	7,894
Estimate of Fair Value Measurement [Member] \| 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,850	7,820
Estimate of Fair Value Measurement [Member] \| 4.55% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	5,903
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member]
Financial Liabilities:
Total long-term debt	231,448	215,271
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	89,315	90,827
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member] \| 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	142,133	124,444
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member]
Financial Assets:
Limited partnership investments	28,944	23,184
Financial Liabilities:
Total long-term debt	30,636	24,941
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,144	9,227
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,739	7,894
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,850	7,820
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 4.55% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	5,903
Carrying Value [Member]
Financial Assets:
Limited partnership investments	28,944	23,184
Financial Liabilities:
Total long-term debt	248,486	237,835
Carrying Value [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	88,216	85,436
Carrying Value [Member] \| 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	129,177	126,454
Carrying Value [Member] \| 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,126	9,270
Carrying Value [Member] \| 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,853	8,206
Carrying Value [Member] \| 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	8,236	$ 8,469
Carrying Value [Member] \| 4.55% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	$ 5,878

Disclosure carrying value of financial assets.

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Disclosure fair value of financial assets.

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Financial Liabilities Carrying Value Disclosure [Abstract]

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Represents the holding of limited partnership interests for investment purposes and is accounted for using the equity method.

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Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=SL6742756-110258

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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