Schedule I - Financial Information of the Registrant (Parent Company) - Balance Sheets (Additional Information) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, par value (USD per share)	$ 1	$ 1
Common stock, shares authorized	50,000,000	50,000,000
Common stock, shares issued	24,071,683	23,783,440
Common stock, shares outstanding	23,719,522	23,431,279
Treasury stock, common shares	352,161	352,161
Parent Company
Condensed Balance Sheet Statements, Captions [Line Items]
Common stock, par value (USD per share)	$ 1	$ 1
Common stock, shares authorized	50,000,000	50,000,000
Common stock, shares issued	23,783,440	22,643,255
Common stock, shares outstanding	23,431,279	22,291,094
Treasury stock, common shares	352,161	352,161
Undistributed earnings	$ 529,417	$ 522,104