Stockholders' Equity - Repurchase (Details)	12 Months Ended
	Dec. 31, 2017 item $ / shares shares	Dec. 31, 2016 $ / shares shares	Oct. 23, 2002 shares
Class of Stock Disclosures
Shares of authorized capital stock	125,000,000
Preferred stock, shares authorized	1,000,000	1,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.10	$ 0.10
Termination period of license after change of ownership of the licensed agent	90 days
Number of shares of outstanding common stock authorized to be repurchased			3,000,000
Purchases made pursuant to repurchase authorization (in shares)	0	0
Number of remaining shares of common stock authorized to be repurchased	1,653,333
Change Of Ownership of a Licensed Agent And Stockholder Rights Plan
Minimum acquisition percentage of the licensed agent to effect change of ownership	10.00%
Minimum transfer percentage of legal or beneficial interest in the licensed agent	20.00%
Minimum acquisition percentage of outstanding common stock by stockholder for which a prior written approval is needed	10.00%
Number of shares of common stock to be purchased for each right held by the registered holder | item	1
Unsolicited acquisition of beneficial ownership as a percentage of outstanding combined common stock and Class A common stock, minimum	10.00%
Tender or exchange offer as a percentage of outstanding combined common stock and Class A common stock, minimum	10.00%
Common Stock | Common Stock
Class of Stock Disclosures
Common stock, shares authorized	74,000,000	74,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.10	$ 0.10
Number of votes per common share held | item	1
Common Stock | Class A Common Stock
Class of Stock Disclosures
Common stock, shares authorized	50,000,000	50,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.10	$ 0.10
Number of votes per common share held | item	10
Conversion ratio of shares	1