Credit Facility (Details) - Line of Credit - USD ($)	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	May 25, 2017
Federal Funds Rate
Line of Credit Facility [Line Items]
Interest rate	0.50%
LIBOR
Line of Credit Facility [Line Items]
Interest rate	1.00%
Base Rate | Minimum
Line of Credit Facility [Line Items]
Interest rate	0.25%
Base Rate | Maximum
Line of Credit Facility [Line Items]
Interest rate	0.75%
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of credit maximum borrowing capacity			$ 300,000,000.0
Possible borrowing capacity increase			150,000,000.0
Line of credit outstanding	$ 79,300,000	$ 116,500,000
Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Commitment fee percentage	0.20%
Fixed charge coverage ratio	1.25
Equity securities limitation, leverage ratio	2.75
Cash availability	$ 25,000,000.0
Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Commitment fee percentage	0.35%
Leverage ratio	3.25
Revolving Credit Facility | LIBOR | Minimum
Line of Credit Facility [Line Items]
Interest rate	1.25%
Revolving Credit Facility | LIBOR | Maximum
Line of Credit Facility [Line Items]
Interest rate	1.75%
Letters of Credit
Line of Credit Facility [Line Items]
Line of credit maximum borrowing capacity			$ 10,000,000.0
Line of credit outstanding	$ 3,200,000	$ 3,200,000
Swingline Loan
Line of Credit Facility [Line Items]
Line of credit maximum borrowing capacity	$ 10,000,000.0