Debt (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 28, 2017 USD ($)
Line of Credit Facility [Line Items]
Maximum borrowing capacity, multiplier of adjusted EBITDA	2.5
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000.0
Maximum additional borrowing capacity	150,000,000.0
Outstanding indebtedness	0	$ 0
Letters of credit, amount outstanding	$ 2,800,000	3,700,000
Total consolidated funded debt to adjusted EBITDA ratio, maximum (not more than)	2.50
EBITDA to interest expense ratio, minimum (not less than)	3.0
Covenant term, minimum liquidity (no less than)			$ 250,000,000.0
Wavier fee			5.00%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Minimum
Line of Credit Facility [Line Items]
Unused capacity, commitment fee	0.25%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Maximum
Line of Credit Facility [Line Items]
Unused capacity, commitment fee	0.375%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Variable Rate Component One | Federal Funds Effective Swap Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.50%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Variable Rate Component One | Eurodollar
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Variable Rate Component One | Eurodollar | Minimum
Line of Credit Facility [Line Items]
Additional spread on variable rate	0.50%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Variable Rate Component One | Eurodollar | Maximum
Line of Credit Facility [Line Items]
Additional spread on variable rate	1.50%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Variable Rate Component Two | Eurodollar | Minimum
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.50%
Revolving Credit Facility | Lines of credit | Revolving Credit Facility Maturing August 2018 | Variable Rate Component Two | Eurodollar | Maximum
Line of Credit Facility [Line Items]
Basis spread on variable rate	2.50%
Letter of Credit | Lines of credit | Revolving Credit Facility Maturing August 2018
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 20,000,000.0
Swingline loans | Lines of credit | Revolving Credit Facility Maturing August 2018
Line of Credit Facility [Line Items]
Maximum borrowing capacity	10,000,000.0
Citibank Credit Facility | Lines of credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity		6,000,000.0
Letters of credit, amount outstanding	$ 2,600,000	$ 2,200,000
Line of credit, expiration period		3 years
Line of credit, fixed interest rate short-term		1.25%
Line of credit, fixed interest rate long-term		1.50%