Divestitures - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Carrying value of investment	$ 34,222		$ 35,403
Gain (loss) on divestiture and sale of investments	$ 561	$ 7,044
Other Sales [Member]
Income Statement Balance Sheet And Additional Disclosures By Disposal Groups Including Discontinued Operations [Line Items]
Carrying value of investment		100
Proceeds from sale of investments		7,100
Gain (loss) on divestiture and sale of investments		$ 7,000