XML 101 R88.htm IDEA: XBRL DOCUMENT v3.8.0.1
Stock-Based Incentive Plans - Movement in Outstanding Shares of Restricted Class A Common Stock and Weighted-Average Grant Date Fair Value Per Share (Detail) - Restricted Class A Common Stock [Member]
9 Months Ended
Sep. 30, 2017
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted Class A Common Stock, Beginning Balance | shares 1,309,995
Restricted Class A Common Stock, Granted | shares 289,896
Restricted Class A Common Stock, Vested | shares (335,536)
Restricted Class A Common Stock, Forfeited | shares (3,123)
Restricted Class A Common Stock, Ending Balance | shares 1,261,232
Weighted-Average Grant Date Fair Value Per Share, Beginning Balance | $ / shares $ 28.68
Weighted-Average Grant Date Fair Value Per Share, Granted | $ / shares 30.55
Weighted-Average Grant Date Fair Value Per Share, Vested | $ / shares 27.78
Weighted-Average Grant Date Fair Value Per Share, Forfeited | $ / shares 27.37
Weighted-Average Grant Date Fair Value Per Share, Ending Balance | $ / shares $ 29.35