Revolving Credit Facility (Details Textual)	9 Months Ended
	Sep. 30, 2015 USD ($) option	Dec. 31, 2014 USD ($)
Line of Credit Facility [Line Items]
Total available borrowings	$ 500,000,000	$ 500,000,000
Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Number of Credit Facility extension options | option	1
Term of Credit Facility extension option	12 months
Amount of available borrowings that can be used for letters of credit and/or surety bonds	$ 100,000,000
Common stock Class A | Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Maximum Class A common stock repurchase amount	100,000,000
Common stock Class A or B [Member] | Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Maximum dividend restriction in any four fiscal quarter period	$ 24,000,000
LIBOR [Member] | Revolving credit facility [Member]
Line of Credit Facility [Line Items]
Variable rate basis spread of debt instrument	3.50%