Fair Value measurements - Summary of Carrying Values and Associated Fair Values of Financial Assets and Liabilities (Detail) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Carrying Value [Member]
Financial assets:
Investments		$ 1,220	$ 37,961
Carrying Value [Member] | 5.75% Senior Notes Due 2028 [Member]
Financial assets:
Senior Notes	[1]	300,000	300,000
Level 2 [Member]
Financial assets:
Investments		1,221	37,958
Level 2 [Member] | 5.75% Senior Notes Due 2028 [Member]
Financial assets:
Senior Notes	[1]	$ 316,215	$ 321,114

[1] Excludes the impact of unamortized debt issuance costs.