Debt - Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Feb. 12, 2019	May 23, 2016
Long-term debt
Total credit facility	$ 300,000		$ 300,000
Amount available	248,200		248,200		$ 248,060
Long-term Line of Credit, Noncurrent	50,000		50,000		50,000
Letters of Credit Outstanding, Amount	1,800		1,800		1,940
Amortization of Debt Issuance Costs	41	$ 46	77	$ 93
Line of Credit | Revolving Credit Facility Due 2024
Long-term debt
Total credit facility					$ 300,000	$ 300,000	$ 300,000
Line of Credit Facility Contingent Increase to Maximum Borrowing Capacity						$ 150,000
Long-term Line of Credit, Noncurrent	$ 50,000		$ 50,000
Weighted-average interest rate	3.54%		3.54%		3.10%
Commitment fee percentage per annum			0.20%		0.20%
Line of Credit | Revolving Credit Facility [Member]
Long-term debt
Long-term Line of Credit, Noncurrent					$ 50,000