Earnings per Share (Details 3) (USD $)	9 Months Ended				3 Months Ended	9 Months Ended				3 Months Ended		3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 TEUs - Amortizing Note [Member]	Sep. 19, 2012 TEUs - Amortizing Note [Member]	Sep. 30, 2012 Tangible Equity Units [Member]	Sep. 30, 2012 Tangible Equity Units [Member]	Sep. 19, 2012 Tangible Equity Units [Member]	Sep. 19, 2012 Tangible Equity Units [Member] Minimum [Member]	Sep. 19, 2012 Tangible Equity Units [Member] Maximum [Member]	Sep. 30, 2012 TEU - Prepaid Purchase Contract [Member]	Sep. 19, 2012 TEU - Prepaid Purchase Contract [Member]	Sep. 30, 2012 Common Class A [Member]	Sep. 30, 2012 Common Class A [Member]	Sep. 19, 2012 Common Class A [Member]
Schedule of Stock Offering [Line Items]
Stock issued during period, shares, new Issues						2,300,000							3,791,004
Sale of stock, price per share														$ 64.75
Shares issued as a result of the underwriter's exercise of their over-allotment option						300,000							525,000
Net proceeds from Class A common stock issuance	$ 234,361,000	$ 0											$ 234,400,000
Shares of stock sold by selling stockholder													233,966
Stated par value per share							$ 100
Net proceeds from TEU issuance	222,856,000	0
Weighted average share impact resulting from offerings					411,307	138,103						453,272	152,194
Coupon rate			5.00%
Maturity date			Oct. 01, 2015
Principal amount per amortizing note			14.1023
Aggregate principal amount			32,400,000
TEU quarterly cash installment per note				$ 1.25
TEU quarterly installment per note on January 1, 2013				$ 1.4167
Annual interest rate				4.50%
TEU number of shares upon conversion per unit								1.2355	1.5444
TEU prepaid stock purchase contract											197,600,000
TEU underwriting discounts and commissions and offering expenses										$ 6,100,000