Office Closing and Exit Costs - Summary of Termination Costs (Detail) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2014	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Severance costs										$ 2,176	$ (822)	$ 11,706
Building lease										(70)	303	3,147
Stock-based compensation										1,664	0	217
Total termination costs	$ (279)	$ (47)	$ (96)	$ 4,192	$ (822)	$ 136	$ 5	$ 162	$ 8,400	$ 3,770	$ (519)	$ 15,070