Long-Term Debt (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Aug. 31, 2018	Dec. 31, 2017	Jul. 06, 2017	Nov. 01, 2016	Dec. 31, 2014	Nov. 11, 2013
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 100,000,000.0			$ 175,000,000.0		$ 250,000,000.0
Allowable Amount for Issuance of Letters of Credit	$ 30,000,000.0					$ 20,000,000.0
Debt Instrument, Covenant, Leverage Ratio	2
Debt Covenant, Minimum Net Income Requirement	$ 1.00
Debt Instrument, Covenant minimum tangible net worth	250,000,000.0
Long-term Line of Credit	0		$ 0
Long-term Debt				$ 17,404,000
Letters of Credit Outstanding on a Line of Credit, Amount Outstanding	10,700,000		3,700,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 89,300,000		$ 171,300,000
Prime Rate [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.00%
Accordion Feature [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000,000.0
Maximum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Commitment Fee Percentage	0.175%
Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.90%
Minimum [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Commitment Fee Percentage	0.0725%
Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.70%