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Fair Value measurements - Summary of Carrying Values and Associated Fair Values of Financial Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Carrying Value [Member]    
Financial assets:    
Investments $ 4,980 $ 10,060
Carrying Value [Member] | 5.75% Senior Notes Due 2028 [Member]    
Financial assets:    
Senior Notes [1] 300,000
Level 2 [Member]    
Financial assets:    
Investments 4,983 10,053
Level 2 [Member] | 5.75% Senior Notes Due 2028 [Member]    
Financial assets:    
Senior Notes [1] $ 309,000
[1] Excludes the impact of unamortized debt issuance costs.