Supplemental Consolidated Balance Sheet Information (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Consolidated Balance Sheet Information [Line Items]
Loans issued	$ 7,500	$ 1,300
Sign-on and retention bonuses issued	8,700	12,400
Purchases of property and equipment	38,650	28,665	$ 39,094
Capital expenditures accrued	3,500
Property and equipment acquired in business acquisitions	100
Retired fully-depreciated assets	21,200
Payments of contingent acquisition liabilities	10,330	828	4,592
Definitive acquisition liabilities related to a small business acquisition	2,000
Contingent acquisition liability adjustments, net	2,213	1,330	(13,047)
Additional paid-in capital recorded through compensation expense	2,547	$ 1,938	1,292
Liability for uncertain tax positions	$ 6,900		$ 1,300
Restricted Stock Units [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Vesting period	3 years
Chicago Office Space [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Deferred rent	$ 4,800
San Francisco Office Spaces [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Deferred rent	2,300
Healthcare [Member] | McKinnis Consulting Services, LLC [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Payments of contingent acquisition liabilities	10,000
Technology Infrastructure and Software [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Purchases of property and equipment	15,200
Leasehold Improvements [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Purchases of property and equipment	18,000
Furniture and Other Equipment [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Purchases of property and equipment	$ 5,400
Maximum [Member]
Supplemental Consolidated Balance Sheet Information [Line Items]
Employee retention and signing bonuses, term, years	6 years
Lease expiration	Dec. 31, 2028