Debt - Long-Term Debt (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Feb. 12, 2019	May 23, 2016
Long-term debt
Total credit facility	$ 300,000		$ 300,000
Amount available	157,000		157,000		$ 198,500
Long-term Line of Credit, Noncurrent	141,300		141,300		99,700
Letters of Credit Outstanding, Amount	1,740		1,740		1,800
Amortization of Debt Issuance Costs	42	$ 41	84	$ 77
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	$ 141,300		$ 141,300
Weighted-average interest rate	2.17%		2.17%		3.25%
Commitment fee percentage per annum			0.25%		0.23%
Line of Credit | Revolving Credit Facility [Member]
Long-term debt
Long-term Line of Credit, Noncurrent					$ 99,700