Stockholders' Equity Scheduled Vesting for Outstanding Restricted Stock Units (Detail) - Restricted Stock Units shares in Millions	Mar. 31, 2017 shares
Schedule of Vesting [Line Items]
Nine Months Ended December 31, 2017	5.9
Year Ended December 31, 2018	7.1
Year Ended December 31, 2019	3.9
Year Ended December 31, 2020	2.0
Year Ended December 31, 2021	0.3
Thereafter	0.3
Total	19.5