Debt - Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Feb. 12, 2019	May 23, 2016
Long-term debt
Total credit facility	$ 300,000		$ 300,000
Amount available	185,500		185,500		$ 248,060
Long-term Line of Credit, Noncurrent	112,700		112,700		50,000
Letters of Credit Outstanding, Amount	1,800		1,800		1,940
Amortization of Debt Issuance Costs	42	$ 46	121	$ 139
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	$ 112,700		$ 112,700
Weighted-average interest rate	3.41%		3.41%		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