Overview and Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Initial offering period			December 2011
Changes in revenue from adjustments of prior period deferred revenue	$ 0.3	$ 0.5	$ 0.3	$ 0.5
Revenue and income from operations		$ 0.2		$ 3.8
Impact on reported earnings per share	$ 0	$ 0	$ 0.01	$ 0.01