Real Estate Charges (Details) (Office Space Closing [Member], USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Sep. 30, 2012 sqft	Dec. 31, 2012	Dec. 31, 2010 Location sqft
Restructuring Cost And Reserve [Line Items]
Area of office space to be sub-leased (in square feet)		510,000		380,000
Portion of office space subleased		30.00%		80.00%
Consolidated number of office locations in New York				2
Number of Locations				3
Real estate charges			$ 202,600,000
Portion of office space to be subleased in New York (in hundredths)		70.00%
Real estate liability restructuring reserve [Abstract]
Balance as of beginning of period	238,784,000		71,164,000
Expense incurred (credit)	648,000		181,589,000
Deferred rent	122,000		27,000,000
Payments made	(21,562,000)		(42,833,000)
Interest accretion	1,137,000		1,864,000
Balance as of end of period	219,129,000		238,784,000
Minimum [Member]
Restructuring Cost And Reserve [Line Items]
Projected Pre-Tax real estate charges as part of workforce reduction		225,000,000
Maximum [Member]
Restructuring Cost And Reserve [Line Items]
Projected Pre-Tax real estate charges as part of workforce reduction		250,000,000