Shareholders' Equity and Long-Term Incentive Compensation - Income (Loss) Per Share (Details) - USD ($) shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Income (loss) from continuing operations	$ 60.0	$ 22.0	$ (8.3)	$ 10.3	$ (3.0)	$ 6.6	$ 6.5	$ 20.2	$ 84.0	$ 30.3	$ (151.6)
Less: Net loss attributable to noncontrolling interests									0.0	(0.4)	(33.4)
Adjustment related to redeemable noncontrolling interest (Note 13)	0.0	0.0	0.0	0.0	0.0	0.0	(18.1)	0.0	0.0	(18.1)	0.0
Income (loss) from continuing operations attributable to SPX Corporation common shareholders for calculating basic and diluted income per share									84.0	12.6	(118.2)
Income (loss) from discontinued operations, net of tax	$ (1.4)	$ 0.3	$ (0.7)	$ 7.1	$ (83.1)	$ (4.7)	$ (3.5)	$ (6.6)	5.3	(97.9)	34.6
Less: Net loss attributable to noncontrolling interest									0.0	0.0	(0.9)
Income (loss) from discontinued operations attributable to SPX Corporation common shareholders for calculating basic and diluted income per share									$ 5.3	$ (97.9)	$ 35.5
Denominator:
Weighted-average number of common shares used in basic income (loss) per share									42,413	41,610	40,733
Dilutive securities — Employee stock options, restricted stock shares and restricted stock units									1,492	551	0
Weighted-average number of common shares and dilutive securities used in diluted income (loss) per share									43,905	42,161	40,733
Unvested Restricted Stock And Restricted Stock Units that did not Meet Required Market Thresholds for Vesting
Stock-based Compensation
Number of awards that were excluded from the computation of diluted income per share									563	1,045	553
Restricted stock shares and restricted stock units
Stock-based Compensation
Number of awards that were excluded from the computation of diluted income per share											351
Stock options
Stock-based Compensation
Number of awards that were excluded from the computation of diluted income per share									997	1,343	505