Long-Term Debt -Term Loan (Details) £ in Millions, $ in Millions	3 Months Ended
	Sep. 30, 2018 GBP (£)	Sep. 30, 2018 USD ($)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2017 GBP (£)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Debt, short-term and long-term						$ 3,303.1	$ 3,267.8
Margin on Borrowings, Reduced Interest Rate from Refinancing in bps					25	25
Net loss on extinguishments of long-term debt			$ (57.4)	$ 0.0
Proceeds from Issuance of Long-term Debt			2,083.0	$ 0.0
Debt Issuance Costs, Gross						$ 2.0
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium						3,280.5	3,234.4
Term Loan [Member]
Debt Instrument [Line Items]
Debt, short-term and long-term						$ 1,479.2	$ 1,483.0
Quarterly amortization payment of original principal, Percent					0.25%	0.25%
Net leverage ratio					2.68	2.68
Long-term Debt, Weighted Average Interest Rate					3.15%	3.15%	3.30%
Amount of Restricted Net Assets for Consolidated and Unconsolidated Subsidiaries						$ 921.0
Debt Instrument, Unamortized Discount						9.0
Deferred Finance Costs, Net						5.0
Deferred Finance Costs and Discount, Net						2.3	$ 14.9
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium						1,476.9	1,468.1
CDW UK Term Loan [Member]
Debt Instrument [Line Items]
Debt, short-term and long-term					£ 56.0	70.2	69.1
Amount of Restricted Net Assets for Consolidated and Unconsolidated Subsidiaries						135.0
Deferred Finance Costs, Net						$ 1.4	1.6
Variable Interest Rate Margin					1.40%	1.40%
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium						$ 68.8	67.5
CDW UK Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt, short-term and long-term						$ 0.0	$ 0.0
Long-term Debt, Weighted Average Interest Rate					0.00%	0.00%	0.00%
Line of Credit Facility, Remaining Borrowing Capacity					£ 50.0	$ 63.0
Long-Term Debt, Net of Deferred Financing Costs, Discount, or Premium						$ 0.0	$ 0.0
Maximum [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Net leverage ratio					3.25	3.25
ABR [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Variable Interest Rate Margin					1.00%	1.00%
LIBOR [Member] | Term Loan [Member]
Debt Instrument [Line Items]
Margin on Borrowings, Reduced Interest Rate from Refinancing in bps					25	25
Variable Interest Rate Margin					2.00%	2.00%
LIBOR [Member] | CDW UK Term Loan [Member]
Debt Instrument [Line Items]
Long-term Debt, Weighted Average Interest Rate					1.76%	1.76%	1.80%
Cap agreement effective January 14, 2015-2017 [Member]
Debt Instrument [Line Items]
Interest Rate Cap, fair value							$ 1.0
Fair Value, Inputs, Level 2 [Member]
Debt Instrument [Line Items]
Debt Instrument, Fair Value Disclosure						$ 3,360.6	$ 3,334.8
Scenario, Forecast [Member] | CDW UK Term Loan [Member]
Debt Instrument [Line Items]
Repayments of Debt	£ 5.0	$ 6.3
Term Loan [Member]
Debt Instrument [Line Items]
Net loss on extinguishments of long-term debt			$ 14.0