Real Estate Charges (Details) ft² in Thousands, $ in Thousands	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2012 ft²	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2010 ft² office_location
Restructuring and Related Activities [Abstract]
Area of office space to be sub-leased (in square feet) | ft²			510				380
Consolidated number of office locations in New York | office_location							2
Number of office locations | office_location							3
Portion of office space subleased (in excess of 90%)			90.00%
Portion of office space to be subleased in New York (more than 70%)			70.00%
Real estate charges (credits)	$ 1,682	$ (980)		$ 1,219	$ 457
Real estate liability restructuring reserve [Abstract]
Balance as of beginning of period				148,429	$ 199,527	$ 199,527
Expense (credit) incurred				2,258		(4,755)
Payments made				(30,877)		(50,893)
Interest accretion				3,232		4,550
Balance as of end of period	$ 123,042			$ 123,042		$ 148,429