Segment information - Operations by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Customer revenue	$ 1,741.0	$ 1,713.2	$ 1,878.4
Gross profit	572.0	483.0	534.0
Total assets	2,419.5	2,707.9	2,504.0
DWS
Segment Reporting Information [Line Items]
Customer revenue	567.0	588.3	641.2
C&I
Segment Reporting Information [Line Items]
Customer revenue	496.5	465.2	527.1
ECS
Segment Reporting Information [Line Items]
Customer revenue	677.5	659.7	710.1
Intersegment Eliminations
Segment Reporting Information [Line Items]
Customer revenue	1.4	0.1	0.0
Intersegment Eliminations | DWS
Segment Reporting Information [Line Items]
Customer revenue	0.0	0.0	0.0
Intersegment Eliminations | C&I
Segment Reporting Information [Line Items]
Customer revenue	0.0	0.0	0.0
Intersegment Eliminations | ECS
Segment Reporting Information [Line Items]
Customer revenue	1.4	0.1	0.0
Operating Segments
Segment Reporting Information [Line Items]
Customer revenue	1,742.4	1,713.3	1,878.4
Gross profit	561.5	454.2	512.9
Depreciation and amortization	129.1	119.3	116.7
Total assets	1,236.6	1,005.3	962.8
Capital expenditures	78.8	83.4	120.2
Operating Segments | DWS
Segment Reporting Information [Line Items]
Customer revenue	567.0	588.3	641.2
Gross profit	76.3	55.3	65.2
Depreciation and amortization	18.5	14.5	18.0
Total assets	352.7	220.7	182.2
Capital expenditures	13.4	13.6	15.2
Operating Segments | C&I
Segment Reporting Information [Line Items]
Customer revenue	496.5	465.2	527.1
Gross profit	56.6	23.2	33.5
Depreciation and amortization	55.1	49.3	54.1
Total assets	290.7	203.6	231.1
Capital expenditures	13.2	24.6	42.1
Operating Segments | ECS
Segment Reporting Information [Line Items]
Customer revenue	678.9	659.8	710.1
Gross profit	428.6	375.7	414.2
Depreciation and amortization	55.5	55.5	44.6
Total assets	593.2	581.0	549.5
Capital expenditures	$ 52.2	$ 45.2	$ 62.9