Special Charges - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Employee	Sep. 30, 2016 USD ($)
Special Charges [Line Items]
Special charges	$ 0	$ 0	$ 30,074,000	$ 6,811,000
Total special charges amount payable	37,600,000		37,600,000
Net non-cash expense reversals			7,500,000
Employee Severance and Other Employee Related Costs
Special Charges [Line Items]
Special charges			$ 16,100,000
Number of employees terminated | Employee			201
Exit Costs Associated with Curtailment of Lease on Executive Office in Washington, D.C.
Special Charges [Line Items]
Special charges			$ 12,400,000
Total special charges amount payable	$ 20,500,000		20,500,000
Non-cash reduction to expense primarily for reversal of deferred rent liability			8,100,000
Other Expenses
Special Charges [Line Items]
Special charges			1,600,000
Non-cash expense			$ 600,000