Long-term Debt (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Feb. 06, 2018 USD ($)	Dec. 31, 2016 USD ($)	Jan. 07, 2016 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility	$ 200,000	$ 200,000
Percentage of subsidiary stock securing debt (in hundredths)		0.65
Unused commitment fee on the average daily unused portion (in hundredths)		0.25%
Average interest rate for the year		2.99%
Letters of Credit Outstanding, Amount	1,000	$ 1,000
Long-term debt	104,400	$ 104,400			$ 104,400	$ 104,400
Special dividend	$ 41,700		$ 50,700
Alternate base rates, applicable margins [Abstract]
Applicable margin, federal funds rate (in hundredths)	0.50%	0.50%
Applicable margin, 30-day LIBOR (in hundredths)	2.00%	2.00%
LIBOR Borrowings [Member]
Line of Credit Facility [Line Items]
Description of basis for variable rate		LIBOR
Applicable margin on variable rate on borrowings, minimum (in hundredths)	1.50%	1.50%
Applicable margin on variable rate on borrowings, maximum (in hundredths)	2.25%	2.25%
Alternate Base Rate Borrowings [Member]
Line of Credit Facility [Line Items]
Description of basis for variable rate		alternate base rate
Applicable margin on variable rate on borrowings, minimum (in hundredths)	0.00%	0.00%
Applicable margin on variable rate on borrowings, maximum (in hundredths)	0.50%	0.50%
Subsequent Event [Member]
Line of Credit Facility [Line Items]
Line of credit facility				$ 350,000
Maximum borrowing capacity				$ 400,000