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Investments - Summary of Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Investment [Line Items]    
Marketable securities $ 75,702 $ 74,907
Parent    
Investment [Line Items]    
Marketable securities 75,702 74,907
Equity method investments 10,570 7,731
Nonqualified retirement plan assets 6,813 5,808
Other investments 3,667 925
Total investments $ 96,752 $ 89,371