Acquisitions, Goodwill and Other Long-Lived Assets (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Oct. 28, 2018	Jul. 25, 2018	Oct. 22, 2017	May 07, 2017	Aug. 29, 2016	Jan. 31, 2016
Segment Reporting Information [Line Items]
Impairment of Intangible Assets, Finite-lived									$ 16,501	$ 16,501
Goodwill, Net	$ 199,092				$ 191,671				199,092	191,671
Payments to Acquire Businesses, Net of Cash Acquired									19,987	23,140	$ 11,800
Unrecognized Tax Benefits, Income Tax Penalties Accrued	61				105				61	105
Unrecognized Tax Benefits, Interest on Income Taxes Accrued	143				201				143	201
Operating Income (Loss)	35,047	$ 29,879	$ 32,870	$ 24,235	27,843	$ 27,176	$ 29,996	$ 23,743	122,031	108,757	59,703
Net income (loss)	$ 27,684	$ 22,329	$ 24,298	$ 17,741	$ 34,681	$ 18,328	$ 19,666	$ 14,581	$ 92,051	$ 87,255	$ 27,505
Basic (in dollars per share)	$ 0.95	$ 0.76	$ 0.83	$ 0.60	$ 1.17	$ 0.61	$ 0.65	$ 0.48	$ 3.14	$ 2.90	$ 0.90
Diluted (in dollars per share)	$ 0.95	$ 0.76	$ 0.82	$ 0.60	$ 1.16	$ 0.61	$ 0.65	$ 0.48	$ 3.12	$ 2.89	$ 0.90
Customer Relationships [Member]
Segment Reporting Information [Line Items]
Impairment of Intangible Assets, Finite-lived									$ 16,501	$ 16,501
Atlantic [Member]
Segment Reporting Information [Line Items]
Business Acquisition, Cash Paid															$ 22,500
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High	$ 135								$ 135
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets															1,821
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Total Tangible Assets															1,821
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles															21,269
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Assets															23,090
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities															590
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities															590
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Net Total Assets															22,500
Atlantic [Member] | Noncompete Agreements [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									5 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill															1,150
Atlantic [Member] | Customer Relationships [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									15 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill															13,400
Atlantic [Member] | Goodwill [Member]
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles															$ 6,719
KCL [Member]
Segment Reporting Information [Line Items]
Business Acquisition, Cash Paid														$ 640
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets														223
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Total Tangible Assets														223
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles														517
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Assets														740
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities														100
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities														100
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Net Total Assets														640
KCL [Member] | Noncompete Agreements [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									2 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill														6
KCL [Member] | Customer Relationships [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									15 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill														234
KCL [Member] | Goodwill [Member]
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles														277
MMT [Member]
Segment Reporting Information [Line Items]
Goodwill, Net	1,954								$ 1,954
Business Acquisition, Cash Paid													$ 3,737
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets													81
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Total Tangible Assets													81
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles													3,656
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Assets													3,737
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Net Total Assets													3,737
MMT [Member] | Noncompete Agreements [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									4 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill													43
MMT [Member] | Customer Relationships [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									15 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill													1,659
MMT [Member] | Goodwill [Member]
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles													$ 1,954
Triumph & Ace [Member]
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets																$ 1,294
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Total Tangible Assets																1,294
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles																11,756
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Assets																13,050
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other																1,250
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities																1,250
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Net Total Assets																11,800
Triumph & Ace [Member] | Noncompete Agreements [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									5 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill																139
Triumph & Ace [Member] | Customer Relationships [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									15 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill																5,335
Triumph & Ace [Member] | Goodwill [Member]
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles																6,282
Ace [Member]
Segment Reporting Information [Line Items]
Business Acquisition, Cash Paid																	$ 1,700
Triumph [Member]
Segment Reporting Information [Line Items]
Business Acquisition, Cash Paid																10,100
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High																$ 1,250
Southwest Freight Distributors [Member]
Segment Reporting Information [Line Items]
Goodwill, Net	$ 5,467								$ 5,467
Business Acquisition, Cash Paid												$ 16,250
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets												933
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Total Tangible Assets												933
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles												15,317
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net Assets												16,250
Business Combination, Recognized Identifiable Assets Acquired and Total Liabilities Assumed, Net Total Assets												16,250
Southwest Freight Distributors [Member] | Noncompete Agreements [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									3 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill												650
Southwest Freight Distributors [Member] | Customer Relationships [Member]
Segment Reporting Information [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life									10 years
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Intangible Assets, Other than Goodwill												9,200
Southwest Freight Distributors [Member] | Goodwill [Member]
Segment Reporting Information [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles												$ 5,467
KCL [Member]
Segment Reporting Information [Line Items]
Business Combination, Contingent Consideration, Liability														$ 100