Fair Value Measurements - Schedule of Fair Value Information for Financial Assets and Liabilities Carried on Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Financial Assets:
Limited partnership investments	$ 23,184	$ 29,263
Financial Liabilities:
Total long-term debt	237,835	138,863
Estimate of Fair Value Measurement [Member]
Financial Assets:
Limited partnership investments	23,184	29,263
Financial Liabilities:
Total long-term debt	240,212	143,484
Revolving credit facility		9,463
Estimate of Fair Value Measurement [Member] \| Senior Notes [Member]
Financial Liabilities:
Total long-term debt		41,618
Estimate of Fair Value Measurement [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	90,827	84,696
Estimate of Fair Value Measurement [Member] \| 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,820	8,664
Estimate of Fair Value Measurement [Member] \| 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,227	8,506
Estimate of Fair Value Measurement [Member] \| 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	124,444
Estimate of Fair Value Measurement [Member] \| 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,894
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member]
Financial Liabilities:
Total long-term debt	215,271	41,618
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member] \| Senior Notes [Member]
Financial Liabilities:
Total long-term debt		41,618
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	90,827
Estimate of Fair Value Measurement [Member] \| (Level 2) [Member] \| 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	124,444
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member]
Financial Assets:
Limited partnership investments	23,184	29,263
Financial Liabilities:
Total long-term debt	24,941	101,866
Revolving credit facility		9,463
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt		84,696
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,820	8,664
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,227	8,506
Estimate of Fair Value Measurement [Member] \| (Level 3) [Member] \| 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,894
Carrying Value [Member]
Financial Assets:
Limited partnership investments	23,184	29,263
Financial Liabilities:
Total long-term debt	237,835	138,863
Revolving credit facility		9,463
Carrying Value [Member] \| Senior Notes [Member]
Financial Liabilities:
Total long-term debt		39,448
Carrying Value [Member] \| 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	85,436	81,988
Carrying Value [Member] \| 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	8,206	8,660
Carrying Value [Member] \| 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	8,469	$ 8,767
Carrying Value [Member] \| 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	126,454
Carrying Value [Member] \| 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	$ 9,270

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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Represents the carrying value of the company's revolving credit facility at balance sheet date.

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Fair value of investment in certain entities that calculate net asset value per share. Includes, but is not limited to, by unit, membership interest, or other ownership interest. Investment includes, but is not limited to, investment in certain hedge funds, venture capital funds, private equity funds, real estate partnerships or funds. Excludes measurement of net asset value as estimate of fair value.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
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Fair value of the amount outstanding under the credit facility.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611

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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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(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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