Office Closing and Exit Costs - Exit Costs Included in Accrued Liabilities in Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Exit costs included in accrued liabilities in consolidated balance sheet
Beginning balance	$ 2,460	$ 11,630
Accrued severance costs	2,176	(822)	$ 11,706
Accrued building rent	(70)	303
Payments	(2,711)	(8,651)
Ending balance	$ 1,855	$ 2,460	$ 11,630