Long-term Debt (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2014 USD ($)	Feb. 06, 2018 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility	$ 200,000			$ 350,000
Maximum borrowing capacity	$ 400,000
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	3.50%
Letters of Credit Outstanding, Amount	$ 1,000
Long-term debt	$ 144,400	$ 104,400
Special dividend		$ 41,700	$ 50,700
Alternate base rates, applicable margins [Abstract]
Applicable margin, federal funds rate (in hundredths)	0.50%
Applicable margin, 30-day LIBOR (in hundredths)	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%
Applicable margin on variable rate on borrowings, maximum (in hundredths)	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%
Applicable margin on variable rate on borrowings, maximum (in hundredths)	0.50%