Investments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments Schedule [Abstract]
Realized gain (loss) on trading securities	$ 0.3	$ 0.1	$ (1.6)	$ (0.3)