Acquisitions - Additional Information (Detail) (USD $)						12 Months Ended		12 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Financial Services [Member]	Sep. 30, 2013 Real Estate Services [Member]	Dec. 31, 2012 Real Estate Services [Member]	Apr. 13, 2012 Grubb & Ellis Company [Member]	Apr. 13, 2012 Grubb & Ellis Company [Member] Real Estate Services [Member]	Dec. 31, 2012 Other Acquisition [Member]	Dec. 31, 2012 Other Acquisition [Member] Financial Services [Member]	Dec. 31, 2012 Other Acquisition [Member] Real Estate Services [Member]	Sep. 30, 2013 Newmark Noncontrolling Interests [Member]	Dec. 31, 2012 Newmark Noncontrolling Interests [Member]	Dec. 31, 2012 Newmark Noncontrolling Interests [Member] Affiliated company of Newmark [Member]
Business Acquisition [Line Items]
Total consideration transferred						$ 47,100,000		$ 24,500,000					$ 2,100,000
Goodwill	163,702,000	164,874,000	1,300,000	78,176,000	79,869,000		5,000,000	20,000,000	4,300,000	15,700,000			1,500,000
Additional noncontrolling interests purchased											9,600,000	8,300,000
Pre-bankruptcy senior secured debt						30,000,000
Notes Receivable						25,600,000
Debtor-in-possession term loans						5,500,000
Cash paid to the bankruptcy estate						16,000,000
Direct costs incurred						$ 2,800,000