Consolidated Statement of Stockholders' Equity - USD ($) $ in Thousands	Total	COMMON STOCK, AMOUNT	PAID-IN CAPITAL	ACCUMULATED OTHER COMPREHENSIVE INCOME(LOSS)	TREASURY STOCK, AT COST	RETAINED EARNINGS	TOTAL IHC STOCKHOLDERS' EQUITY	NON-CONTROLLING INTERESTS
Treasury Stock, Shares					(1,160,118)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Beginning Balance at Dec. 31, 2014	$ 309,612	$ 18,531	$ 127,098	$ 22	$ (12,141)	$ 166,177	$ 299,687	$ 9,925
Common Stock, Shares, Outstanding, Beginning Balance at Dec. 31, 2014		18,531,158
Net Income (Loss), Including Portion Attributable to Nonredeemable Noncontrolling Interest	30,522					29,944	29,944	578
Other Comprehensive Income (Loss), Net of Tax, Total	(3,462)			(3,467)			(3,467)	5
Treasury Stock, Value, Acquired, Cost Method	(1,820)				$ (1,820)		(1,820)	0
Treasury Stock, Shares, Acquired					(143,307)
Acquisition of subsidiary	608						0	608
Purchases of noncontrolling interests	(1,734)		112	5			117	(1,851)
Dividends, Common Stock, Cash	(1,562)					(1,562)	(1,562)	0
Sharebased Compensation Effect On Stockholders Equity	446	$ 38	408				446	0
Sharebased Compensation Effect On Stockholders Equity, Shares Issued		38,025
Other capital transactions	94		115			(109)	6	88
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance at Dec. 31, 2015	332,704	$ 18,569	127,733	(3,440)	$ (13,961)	194,450	323,351	9,353
Common Stock, Shares, Outstanding, Ending Balance at Dec. 31, 2015		18,569,183
Treasury Stock, Shares					(1,303,425)
Net Income (Loss), Including Portion Attributable to Nonredeemable Noncontrolling Interest	133,314					123,298	123,298	10,016
Other Comprehensive Income (Loss), Net of Tax, Total	(3,508)			(3,626)			(3,626)	118
Treasury Stock, Value, Acquired, Cost Method	(3,522)				$ (3,522)		(3,522)	0
Treasury Stock, Shares, Acquired					(214,558)
Purchases of noncontrolling interests	(18,111)		(2,302)	102			(2,200)	(15,911)
Dividends, Common Stock, Cash	(1,800)					(1,800)	(1,800)	0
Sharebased Compensation Effect On Stockholders Equity	778	$ 51	727				778	0
Sharebased Compensation Effect On Stockholders Equity, Shares Issued		51,325
Other capital transactions	348		310			(30)	280	68
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance at Dec. 31, 2016	$ 439,256	$ 18,620	126,468	(6,964)	$ (17,483)	315,918	436,559	2,697
Common Stock, Shares, Outstanding, Ending Balance at Dec. 31, 2016	17,102,525	18,620,508
Distributions to noncontrolling interests	$ (947)						0	(947)
Treasury Stock, Shares	(1,517,983)				(1,517,983)
Net Income (Loss), Including Portion Attributable to Nonredeemable Noncontrolling Interest	$ 42,069					42,042	42,042	27
Other Comprehensive Income (Loss), Net of Tax, Total	3,181			3,181			3,181	0
Treasury Stock, Value, Acquired, Cost Method	$ (46,527)				$ (46,527)		(46,527)	0
Treasury Stock, Shares, Acquired	(2,289,502)				(2,289,502)
Dividends, Common Stock, Cash	$ (2,392)					(2,392)	(2,392)	0
Sharebased Compensation Effect On Stockholders Equity	(1,319)	$ 5	(1,930)		$ 606		(1,319)	0
Sharebased Compensation Effect On Stockholders Equity, Shares Issued		4,950			72,312
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance at Dec. 31, 2017	$ 434,243	$ 18,625	$ 124,538	(4,598)	$ (63,404)	356,383	431,544	2,699
Common Stock, Shares, Outstanding, Ending Balance at Dec. 31, 2017	14,890,285	18,625,458
Distributions to noncontrolling interests	$ (25)						0	$ (25)
Stockholders Equity, Reclassification Of Stranded Tax Effects In Accumulated Other Comprehensive Income	$ 0			$ (815)		$ 815	$ 0
Treasury Stock, Shares	(3,735,173)				(3,735,173)