Shared-Based Compensation and Share Repurchases Plans (Narrative) (Details) - USD ($)			3 Months Ended	12 Months Ended
	Nov. 07, 2017	May 16, 2014	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Mar. 02, 2017
Equity And Stock-Based Compensation [Line Items]
Unrecognized estimated compensation cost related to non-vested stock options granted			$ 878,000	$ 878,000
Recognition period of unrecognized compensation cost				1 year 10 months 28 days
Intrinsic unrealized value of all options outstanding, vested and expected to vest			2,202,772,000	$ 2,202,772,000	$ 2,584,500,000	$ 1,401,321,000
Proceeds from the exercise of stock option				52,000	146,000	492,000
Stock Option [Member]
Equity And Stock-Based Compensation [Line Items]
Compensation expense				310,000	284,000	282,000
Proceeds from the exercise of stock option				574,000	146,000	$ 3,000,000
Restricted Stock Units (RSUs) [Member]
Equity And Stock-Based Compensation [Line Items]
Compensation expense				668,000	$ 396,000
Unrecognized estimated compensation cost related to non-vested stock options granted			852,000	$ 852,000
Restricted Stock Units (RSUs) [Member] | Director [Member]
Equity And Stock-Based Compensation [Line Items]
Vesting period of stock options and RSU				1 year
Percentage of shares vested				100.00%
Restricted Stock Units (RSUs) [Member] | Management [Member]
Equity And Stock-Based Compensation [Line Items]
Vesting period of stock options and RSU				4 years
Percentage of shares vested				25.00%
2014 Stock Repurchase Plan [Member]
Equity And Stock-Based Compensation [Line Items]
Repurchase program, amount authorized		$ 10,000,000
Repurchase program, shares authorized		350,000
Share price		$ 11.74
Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Repurchase program, remaining amount authorized			18,500,000	$ 18,500,000
Shares granted				169,762	169,327	112,000
Class A [Member] | 2017 Stock Repurchase Plan [Member]
Equity And Stock-Based Compensation [Line Items]
Repurchase value			$ 6,500,000	$ 6,500,000
Repurchase program, amount authorized								$ 25,000,000
Share price				$ 15.92
Class A [Member] | James Cotter Jr. [Member]
Equity And Stock-Based Compensation [Line Items]
Stock options terminated				50,000
2010 Stock Incentive Plan [Member]
Equity And Stock-Based Compensation [Line Items]
Additional shares authorized	947,460
Common Stock shares remaining for future issuances							302,540
Repurchase program, shares authorized							302,540
2010 Stock Incentive Plan [Member] | Class A Nonvoting Common Stock [Member]
Equity And Stock-Based Compensation [Line Items]
Common Stock authorized for issuance under 2010 Stock Incentive Plan			1,250,000	1,250,000
Common Stock shares remaining for future issuances			1,250,000	1,250,000
2010 Stock Incentive Plan [Member] | Class A [Member]
Equity And Stock-Based Compensation [Line Items]
Shares granted			0
2010 Stock Incentive Plan [Member] | Minimum [Member]
Equity And Stock-Based Compensation [Line Items]
Vesting period of stock options and RSU				0 years
2010 Stock Incentive Plan [Member] | Maximum [Member]
Equity And Stock-Based Compensation [Line Items]
Vesting period of stock options and RSU				4 years