Employee Incentive and Retirement Plans - Total Stock-Based Compensation Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 70,983	$ 69,201	$ 51,222
Stock issued related to acquisition
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	159	2,575	9,416
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	15,103	23,203	30,717
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	54,116	41,737	9,185
Available for ESPP
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,605	$ 1,686	$ 1,904