Investments - Summary of Investments (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Investment [Line Items]
Marketable securities	$ 80,234	$ 41,496
Total investments	567,018	579,558
Parent [Member]
Investment [Line Items]
Marketable securities	80,234	41,496
Equity method investments	5,147	9,007
Nonqualified retirement plan assets	5,430	5,310
Other investments	925	925
Total investments	$ 91,736	$ 56,738