Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended													12 Months Ended
	Dec. 31, 2015		Sep. 30, 2015	Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014		Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Impairment charges and other	$ (600)			$ 1,300		$ 9,800								$ 10,480	$ 3,687	$ 3,342
Selected Quarterly Financial Information [Abstract]
Revenues	650,011		$ 626,286	627,838		699,479		$ 595,865		$ 560,303	$ 564,621		$ 636,999	2,603,614	2,357,788	2,256,112
Gross profit	97,045		106,743	104,219		129,860		101,359		100,817	95,453		106,176	437,867	403,805	393,251
Operating income	10,026	[1]	19,936	12,217	[2]	23,520	[3]	13,009	[4]	14,460	3,414	[5]	16,591	65,699	47,474	56,223
Net income	$ 6,339		$ 11,950	$ 7,314		$ 13,787		$ 8,164		$ 8,385	$ 1,891		$ 9,564	$ 39,390	$ 28,004	$ 32,032
Basic net income per share of common stock (in dollars per share)	$ 0.26		$ 0.48	$ 0.29		$ 0.54		$ 0.27	[6]	$ 0.33	$ 0.07		$ 0.37	$ 1.58	$ 1.05	$ 1.25
Diluted net income per share (in dollars per share)	$ 0.26		$ 0.48	$ 0.29		$ 0.54		$ 0.27	[6]	$ 0.33	$ 0.07		$ 0.37	$ 1.58	$ 1.05	$ 1.25
Goodwill and Intangible Asset Impairment													$ 2,500			$ 3,300
Asset impairment charge due to change in office consolidation plans								$ 1,200

[1] Includes a reduction to non-cash impairment and other charges in the fourth quarter of 2015 of $0.6 million. Please read Note 6, “Other Asset Impairments,” for additional information.

[2] Includes non-cash impairment and other charges in the second quarter of 2015 of $1.3 million. Please read Note 6, “Other Asset Impairments,” for additional information.

[3] Includes non-cash impairment and other charges in the first quarter of 2015 of $9.8 million. Please read Note 6, “Other Asset Impairments,” for additional information.

[4] Includes a $1.2 million non-cash charge in the fourth quarter of 2014. Please read Note 1, “Accounting Policies,” for additional information.

[5] Includes a non-cash impairment charge in the first quarter of 2014 of $2.5 million. Please read Note 5, “Goodwill and Other Intangible Assets,” for additional information.

[6] Includes the impact of dividends exceeding earnings under the two-class method, resulting in a $0.05 earnings per share decrease in the fourth quarter of 2014. Please read Note 11, “Net Income Per Share,” for additional information.