Income Taxes (Schedule of Deferred Income Taxes) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Allowance for uncollectible receivables	$ 4,600	$ 5,231
Deferred revenue	3,284	3,427
Accrued compensation	1,884	1,601
Accrued office consolidation costs	224	644
Interest rate derivatives		26
Share-based compensation	8,072	10,403
Unsecured employee loans	1,055	785
Deferred rent	4,806	8,683
Foreign net operating losses	1,723	1,417
Other	854	2,171
Deferred tax assets before foreign valuation allowance	26,502	34,388
Foreign valuation allowance	(1,729)	(1,410)
Deferred tax assets	24,773	32,978
Depreciation and amortization	(10,370)	(10,965)
Prepaid expenses	(1,964)	(3,422)
Deferred tax liabilities	(85,904)	(110,715)
Net deferred tax liabilities	(61,131)	(77,737)
Domestic [Member]
Income Taxes [Line Items]
Goodwill and intangible assets — acquisition cost	(72,918)	(95,708)
Foreign [Member]
Income Taxes [Line Items]
Goodwill and intangible assets — acquisition cost	$ (652)	$ (620)