Calculations of Net Income (Loss) Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Basic and Diluted:
Income (loss) from continuing operations	$ 15,272		$ 23,192		$ (5,764)		$ (6,488)		$ 18,291		$ (2,324)		$ (2,519)		$ 16,565		$ 26,212	$ 30,013	$ 48,254
Income (loss) from discontinued operations	(610)		(1,851)		(230)		(532)		(425)		(633)		(714)		(675)		(3,223)	(2,447)	(1,665)
Net income (loss) attributable to ICG Group, Inc.	$ 14,662		$ 21,341		$ (5,994)		$ (7,020)		$ 17,866		$ (2,957)		$ (3,233)		$ 15,890		$ 22,989	$ 27,566	$ 46,589
Basic:
Income (loss) from continuing operations per share	$ 0.43		$ 0.65		$ (0.16)		$ (0.18)		$ 0.50		$ (0.06)		$ (0.07)		$ 0.45		$ 0.73	$ 0.82	$ 1.33
Income (loss) from discontinued operations per share	$ (0.02)		$ (0.05)		$ (0.01)		$ (0.01)		$ (0.01)		$ (0.02)		$ (0.02)		$ (0.02)		$ (0.09)	$ (0.07)	$ (0.05)
Net income (loss) attributable to ICG Group, Inc. per share	$ 0.41		$ 0.60		$ (0.17)		$ (0.19)		$ 0.49		$ (0.08)		$ (0.09)		$ 0.43		$ 0.64	$ 0.75	$ 1.28
Diluted:
Income (loss) from continuing operations per share	$ 0.42		$ 0.64		$ (0.16)		$ (0.18)		$ 0.50		$ (0.06)		$ (0.07)		$ 0.44		$ 0.72	$ 0.80	$ 1.30
Income (loss) from discontinued operations per share	$ (0.02)		$ (0.05)		$ (0.01)		$ (0.01)		$ (0.01)		$ (0.02)		$ (0.02)		$ (0.02)		$ (0.09)	$ (0.06)	$ (0.04)
Net income (loss) attributable to ICG Group, Inc. per share	$ 0.40		$ 0.59		$ (0.17)		$ (0.19)		$ 0.49		$ (0.08)		$ (0.09)		$ 0.42		$ 0.63	$ 0.74	$ 1.26
Shares used in computation of basic income (loss) per share	35,840	[1]	35,650	[1]	35,917	[1]	36,156	[1]	36,170	[1]	36,556	[1]	36,961	[1]	36,944	[1]	35,890	36,656	36,427
Incremental Diluted Shares Impact:
Shares used in the computation of diluted income (loss) per share	36,912	[1]	36,273	[1]	35,917	[1]	36,156	[1]	36,633	[1]	36,556	[1]	36,961	[1]	37,991	[1]	36,543	37,460	37,064
Stock Options
Incremental Diluted Shares Impact:
Number of dilutive shares																	41	84	65
SARs
Incremental Diluted Shares Impact:
Number of dilutive shares																	546	683	538
Restricted Stock
Incremental Diluted Shares Impact:
Number of dilutive shares																	40	20	19
DSUs
Incremental Diluted Shares Impact:
Number of dilutive shares																	26	17	15

[1] The sum of quarterly income (loss) per share differs from the full year amount due to changes in the number of shares outstanding during the year.