Fair Value Measurements (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended
	Feb. 01, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Accrued earnouts		$ 400		$ 500
Performance targets, maximum potential obligation		10,300		13,300
Treasury securities		200		200
Held-to-maturity investments, carrying value		200		200
Held-to-maturity investments, fair value		200		200
Purchase of investments		150,053	$ 686
Proceeds from sale of investments		150,557
Business Combinations and Asset Acquisitions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Performance targets, maximum potential obligation		600		$ 700
2018 Term Loan [Member] | Interest Rate Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loan period	4 years
Notional amount	$ 1,200,000
Derivative basis spread on variable interest rate	2.00%
Derivative fixed interest rate	4.495%
Liability fair value of swap		$ 15,300