Condensed Consolidated Statement Of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Additional Paid-in Capital [Member]	Retained Earnings [Member]	Total Knight-Swift Equity [Member]	Noncontrolling Interest [Member]	Common Class A [Member]
Beginning balance, shares at Dec. 31, 2017						177,998
Beginning balance, value at Dec. 31, 2017	$ 5,240,370	$ 4,219,214	$ 1,016,738	$ 5,237,732	$ 2,638	$ 1,780
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued to employees (shares)						285
Common stock issued to employees	4,090	4,087		4,090		$ 3
Common stock issued under employee stock purchase plan (shares)						9
Common stock issued under employee stock purchase plan	371	371		371		$ 0
Shares withheld – restricted stock unit settlement	(2,118)		(2,118)	(2,118)
Employee stock-based compensation expense	1,989	1,989		1,989
Cash dividends paid and dividends accrued ($0.06 per share)	(10,742)		(10,742)	(10,742)
Net income attributable to Knight-Swift	70,364		70,364	70,364
Distribution to noncontrolling interest	(801)				(801)
Net income attributable to noncontrolling interest	368				368
Net cumulative-effect adjustment from adopting ASC Topic 606	5,301		5,301	5,301
Ending balance, shares at Mar. 31, 2018						178,292
Ending balance, value at Mar. 31, 2018	5,309,192	4,225,661	1,079,543	5,306,987	2,205	$ 1,783
Beginning balance, shares at Dec. 31, 2018						172,844
Beginning balance, value at Dec. 31, 2018	5,462,719	4,242,369	1,216,852	5,460,949	1,770	$ 1,728
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock issued to employees (shares)						198
Common stock issued to employees	2,375	2,373		2,375		$ 2
Common stock issued under employee stock purchase plan (shares)						24
Common stock issued under employee stock purchase plan	566	566		566		$ 0
Shares withheld – restricted stock unit settlement	(1,514)		(1,514)	(1,514)
Employee stock-based compensation expense	2,880	2,880		2,880
Cash dividends paid and dividends accrued ($0.06 per share)	(10,438)		(10,438)	(10,438)
Net income attributable to Knight-Swift	87,938		87,938	87,938
Distribution to noncontrolling interest	(148)				(148)
Net income attributable to noncontrolling interest	245				245
Ending balance, shares at Mar. 31, 2019						173,066
Ending balance, value at Mar. 31, 2019	$ 5,544,623	$ 4,248,188	$ 1,292,838	$ 5,542,756	$ 1,867	$ 1,730