Debt (Details)	Jul. 02, 2018 USD ($)	Apr. 15, 2018	Mar. 16, 2018	Feb. 15, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Amount outstanding					$ 338,874,000	$ 248,320,000
Additional Term Loan 2018 | Term loan
Line of Credit Facility [Line Items]
Borrowing capacity				$ 105,000,000
Covenant terms, leverage ratio				2.50
Covenant terms, percent of aggregate revolving commitments				30.00%
Credit Facility
Line of Credit Facility [Line Items]
Period after effective date		60 days	31 days
Delayed draw fee, percent		2.50%	1.25%
Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Basis spread on variable interest rate				2.50%
Basis spread on variable rate, reduction in rate				0.25%
Potential reduction to basis spread on variable rate, pro forma leverage ratio used for calculation				1.00
Credit Facility | Alternate base rate
Line of Credit Facility [Line Items]
Basis spread on variable interest rate				1.50%
Credit Facility | Term loan
Line of Credit Facility [Line Items]
Amount outstanding					351,500,000
Credit Facility | Revolving credit facility
Line of Credit Facility [Line Items]
Borrowing capacity					100,000,000
Amount outstanding					$ 0
Sustainable Growth Advisers, LP
Line of Credit Facility [Line Items]
Term loan debt drawn at closing	$ 105,000,000