Condensed Consolidated Statement Of Stockholders' Equity (Unaudited) - 3 months ended Mar. 31, 2018 - 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]
Issuance of common stock to employees (shares)						285
Issuance of common stock to employees	4,090	4,087		4,090		$ 3
Shares issued under employee stock purchase plan (shares)						9
Shares 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	(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
Cumulative-effect adjustment from adopting ASC 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