Debt - Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Feb. 12, 2019	May 23, 2016
Long-term debt
Total credit facility	$ 300,000
Amount available	248,060		$ 248,060
Long-term Line of Credit, Noncurrent	50,000		50,000
Letters of Credit Outstanding, Amount	1,940		1,940
Amortization of Debt Issuance Costs	36	$ 46
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
Weighted-average interest rate	3.60%		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