Revolving Credit Facility - Narrative (Details)			12 Months Ended
	Feb. 22, 2016	Aug. 02, 2010 USD ($)	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date					Sep. 25, 2019
Long-term Line of Credit, Noncurrent					$ 0	$ 128,398,000
Line of Credit, Current					142,736,000	0
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums					$ 700,000	$ 400,000
August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Ratio of indebtedness to net capital					3.5
Maximum leverage ratio					4.5
Debt instrument converstion ratio					5.0
Minimum leverage ratio					4.0
Revolving Credit Facility | Credit Agreement Due 2019 [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity on line of credit		$ 175,000,000
Revolving Credit Facility | August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Increase to borrowing capacity		$ 50,000,000
Unused capacity					$ 6,000,000
Borrowings to be drawn					$ 700,000
Maximum leverage ratio		3.0
Minimum interest coverage ratio		5
Revolving Credit Facility | Credit Agreement Due 2020 [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity on line of credit		$ 160,000,000
Base Rate
Line of Credit Facility [Line Items]
Additional basis on borrowings	110.00%
Minimum
Line of Credit Facility [Line Items]
Additional basis on borrowings					5000.00%
Minimum | Eurodollar
Line of Credit Facility [Line Items]
Additional basis on borrowings					15000.00%
Minimum | Eurodollar | Revolving Credit Facility | August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Additional basis on borrowings		1.25%
Minimum | Base Rate | Revolving Credit Facility | August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Additional basis on borrowings		0.25%
Maximum
Line of Credit Facility [Line Items]
Additional basis on borrowings					22500.00%
Maximum | Eurodollar
Line of Credit Facility [Line Items]
Additional basis on borrowings					32500.00%
Maximum | Eurodollar | Revolving Credit Facility | August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Additional basis on borrowings		2.50%
Maximum | Base Rate | Revolving Credit Facility | August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Additional basis on borrowings		1.50%
International [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity on line of credit					$ 5,000,000
Unused capacity					$ 4,500,000
Subsequent Event [Member] | August 2010 Credit Facility Agreement
Line of Credit Facility [Line Items]
Ratio of indebtedness to net capital				3.00
Maximum leverage ratio			3.0	4.75
Debt instrument converstion ratio				5.0
Minimum leverage ratio			5.00	3.50