Significant Accounting Policies (Share-based Compensation and Other) (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of directors, annual retainer paid in restricted shares	$ 22,500
Annual non-employee directors retainer amount	$ 37,500		$ 37,500
Class A Common Stock | Executives | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued during the year under the Company's stock compensation plans (shares)			92,572	62,425
Class A common stock available for issuance under the plan (shares)	407,428		407,428
Compensation expense related to share awards			$ 3,500,000	$ 4,300,000	$ 1,700,000
Compensation expense related to share awards, net of tax			$ 2,300,000	2,800,000	1,100,000
Class A Common Stock | Non-employee directors | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Class A common stock available for issuance under the plan (shares)	81,570		81,570
Compensation expense related to share awards			$ 900,000	900,000	700,000
Compensation expense related to share awards, net of tax			$ 600,000	$ 600,000	$ 500,000
Class A Common Stock | Non-employee directors | Restricted stock
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued during the year under the Company's stock compensation plans (shares)			18,643	10,690	11,496
Amount of directors, annual retainer paid in restricted shares		$ 66,750		$ 82,000	$ 75,000
Annual non-employee directors retainer amount		$ 108,750		$ 138,000	$ 131,000
Percentage of annual retainer that may be received in shares of Class A stock (percent)	100.00%		100.00%
Class A Common Stock | Non-employee directors | Voluntary shares
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued during the year under the Company's stock compensation plans (shares)			2,746	2,282	2,553
Class A Common Stock | Participant's Retirement Date | Executives | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Duration of restrictions on stock assignment, pledges or transfers			5 years
Class A Common Stock | Date of award | Executives | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Duration of restrictions on stock assignment, pledges or transfers			10 years
Class A Common Stock | Date of award | Non-employee directors | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Duration of restrictions on stock assignment, pledges or transfers			10 years
Class A Common Stock | Participants retirement from board of directors | Non-employee directors | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Duration of restrictions on stock assignment, pledges or transfers			5 years
Class A Common Stock | Minimum age of director upon retirement from board | Non-employee directors | Stock Compensation Plan [Member]
Schedule of Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Duration of restrictions on stock assignment, pledges or transfers			70 years