Long-term Debt (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Current borrowing capacity	$ 350,000
Maximum borrowing capacity	$ 400,000
Percentage Of Subsidiary Stock Securing Debt	0.65
Long-term debt	$ 104,400	$ 104,400
Letters of Credit Outstanding, Amount	1,000
Line of Credit Facility, Remaining Borrowing Capacity	$ 244,600
Applicable Margin Federal Funds Rate	0.50%
Applicable Margin 30 Day Libor	2.00%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.25%
Line of Credit Facility, Interest Rate During Period	3.59%
London Interbank Offered Rate (LIBOR) [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.50%
London Interbank Offered Rate (LIBOR) [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
Base Rate [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.00%
Base Rate [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.50%