Fair Value Measurements - Schedule of Fair Value Information for Financial Assets and Liabilities Carried on Balance Sheet (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Financial Assets:
Limited partnership investments	$ 32,293	$ 23,184
Estimate of Fair Value Measurement [Member]
Financial Assets:
Limited partnership investments	32,293	23,184
Financial Liabilities:
Total long-term debt	266,383	240,212
Estimate of Fair Value Measurement [Member] | 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	89,824	90,827
Estimate of Fair Value Measurement [Member] | 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	145,617	124,444
Estimate of Fair Value Measurement [Member] | 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,128	9,227
Estimate of Fair Value Measurement [Member] | 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,788	7,894
Estimate of Fair Value Measurement [Member] | 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	8,001	7,820
Estimate of Fair Value Measurement [Member] | 4.55% Promissory note [Member]
Financial Liabilities:
Total long-term debt	6,025
Estimate of Fair Value Measurement [Member] | (Level 2) [Member]
Financial Liabilities:
Total long-term debt	235,441	215,271
Estimate of Fair Value Measurement [Member] | (Level 2) [Member] | 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	89,824	90,827
Estimate of Fair Value Measurement [Member] | (Level 2) [Member] | 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	145,617	124,444
Estimate of Fair Value Measurement [Member] | (Level 3) [Member]
Financial Assets:
Limited partnership investments	32,293	23,184
Financial Liabilities:
Total long-term debt	30,942	24,941
Estimate of Fair Value Measurement [Member] | (Level 3) [Member] | 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,128	9,227
Estimate of Fair Value Measurement [Member] | (Level 3) [Member] | 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,788	7,894
Estimate of Fair Value Measurement [Member] | (Level 3) [Member] | 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	8,001	7,820
Estimate of Fair Value Measurement [Member] | (Level 3) [Member] | 4.55% Promissory note [Member]
Financial Liabilities:
Total long-term debt	6,025
Carrying Value [Member]
Financial Assets:
Limited partnership investments	32,293	23,184
Financial Liabilities:
Total long-term debt	250,095	237,835
Carrying Value [Member] | 3.875% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	89,181	85,436
Carrying Value [Member] | 4.25% Convertible Senior Notes [Member]
Financial Liabilities:
Total long-term debt	130,120	126,454
Carrying Value [Member] | 3.95% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	9,077	9,270
Carrying Value [Member] | 4% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	7,732	8,206
Carrying Value [Member] | 3.75% Promissory Note [Member]
Financial Liabilities:
Total long-term debt	8,159	$ 8,469
Carrying Value [Member] | 4.55% Promissory note [Member]
Financial Liabilities:
Total long-term debt	$ 5,826