Debt - Amended credit agreement (Details)		3 Months Ended		9 Months Ended
	Apr. 02, 2018 USD ($)	Sep. 30, 2018 USD ($) Lender	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Lender	Sep. 30, 2017 USD ($)	Oct. 26, 2018	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Loss on extinguishment of debt, unamortized debt issuance costs		$ 0	$ 0	$ 748,000	$ 842,000
Interest coverage ratio				3
Maximum leverage ratio		4.25		4.25
Line of credit | London Interbank Offered Rate (LIBOR) | Minimum | United States of America, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.25%
Line of credit | London Interbank Offered Rate (LIBOR) | Maximum | United States of America, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	4.00%
Line of credit | Base rate | Minimum | United States of America, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	1.25%
Line of credit | Base rate | Minimum | Canada, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	1.25%
Line of credit | Base rate | Maximum | United States of America, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	3.00%
Line of credit | Base rate | Maximum | Canada, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	3.00%
Line of credit | Canadian Dealer Offered Rate (CDOR) | Minimum | Canada, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.25%
Line of credit | Canadian Dealer Offered Rate (CDOR) | Maximum | Canada, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	4.00%
Line of credit | Bank Bill Swap Bid Rate (BBSY) | Minimum | Australia, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	2.25%
Line of credit | Bank Bill Swap Bid Rate (BBSY) | Maximum | Australia, Dollars
Line of Credit Facility [Line Items]
Debt instrument, basis spread on variable rate	4.00%
Amended credit facility
Line of Credit Facility [Line Items]
Number of lenders | Lender		9		9
Amended credit facility | Minimum
Line of Credit Facility [Line Items]
Lender commitments, within credit agreement		$ 24,900,000		$ 24,900,000
Amended credit facility | Maximum
Line of Credit Facility [Line Items]
Lender commitments, within credit agreement		110,600,000		$ 110,600,000
Amended credit facility | US term loan
Line of Credit Facility [Line Items]
Term loan, face amount							$ 350,000,000
Amortization on term loan facility	10.00%
Revolving credit facility | Amended credit facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 239,500,000						275,000,000
Reduction of aggregate revolving loan commitments, maximum borrowing capacity	35,500,000
Revolving credit facility, U.S. subsidiaries | Amended credit facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	20,000,000						40,000,000
Revolving credit facility, Canadian subsidiaries | Amended credit facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	159,500,000						90,000,000
Revolving credit facility, the Company | Amended credit facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity							60,000,000
Revolving credit facility, Australian subsidiaries | Amended credit facility
Line of Credit Facility [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 60,000,000						$ 85,000,000
Loss on extinguishment of debt, unamortized debt issuance costs		$ 700,000
Subsequent Event [Member] | Amended credit facility | US term loan
Line of Credit Facility [Line Items]
Amortization on term loan facility						12.50%