Share-Based Employee Compensation - Shares Authorized, Available for Future Grant and Outstanding Under Each Plans (Detail)	Dec. 31, 2017 shares
Equity Incentive Plan [Line Items]
Authorized (in shares)	34,000,000
Available (in shares)	11,622,000
Outstanding (in shares)	3,695,000
2004 Plan
Equity Incentive Plan [Line Items]
Authorized (in shares)	33,000,000
Available (in shares)	10,875,000
Outstanding (in shares)	3,635,000
2013 Directors Plan
Equity Incentive Plan [Line Items]
Authorized (in shares)	1,000,000
Available (in shares)	747,000
Outstanding (in shares)	60,000