Revolving Credit Facility (Details Textual) (USD $)	6 Months Ended			6 Months Ended			0 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Revolving credit facility [Member]	Jun. 30, 2014 Common stock Class A [Member] Revolving credit facility [Member]	Jun. 30, 2014 Common stock Class A or B [Member] Revolving credit facility [Member]	Jul. 01, 2014 Subsequent Event [Member] Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Total available borrowings	$ 500,000,000		$ 500,000,000
Credit Facility maturity terms				February 21, 2016 and provides for one, 12-month extension option
Number of Credit Facility extension options				1
Term of Credit Facility extension option				12 months
Credit Facility interest rate				LIBOR plus 3.50%
Variable rate basis of debt instrument				LIBOR
Variable rate basis spread of debt instrument				3.50%
Amount of available borrowings that can be used for letters of credit and/or surety bonds				100,000,000
Maximum Class A common stock repurchase amount					100,000,000
Maximum dividend restriction in any four fiscal quarter period						24,000,000
Repayment of Credit Facility	$ 329,825,000	$ 86,950,000					$ 196,000,000