Debt Credit Facility (Details Textual) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, current borrowing capacity	$ 350.0
Credit facility additional borrowing capacity available	250.0
Credit facility, maximum borrowing capacity	$ 600.0
Credit facility, maturity date	Jan. 28, 2022
Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, commitment fee percentage	0.15%
Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, commitment fee percentage	0.30%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, interest rate spread	1.25%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, interest rate spread	1.875%
Base Rate [Member] | Minimum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, interest rate spread	0.25%
Base Rate [Member] | Maximum [Member] | Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, interest rate spread	0.875%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit facility, covenant, debt to EBITDA ratio	3
Credit facility, covenant, debt to EBITDA ratio after step up triggered by acquisition	3.50
Credit facility, covenant, required business acquisition consideration, minimum	$ 250.0
Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Credit facility, covenant, pro forma leverage ratio	2.75
Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Credit facility, covenant, pro forma leverage ratio	3.25