Fair Value Remeasurement Based on Significant Inputs Not Observable (Parenthetical) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of liabilities settlements	[1]	$ 1.5	$ 7.6
Common Stock
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value of liabilities settlements		$ 1.2	$ 6.6

[1] Acquisition liabilities settled in common stock totaled $1.2 million and $6.6 million during 2017 and 2016, respectively.