Capital Stock (Detail Textuals) (USD $)	1 Months Ended	2 Months Ended	3 Months Ended		12 Months Ended	1 Months Ended	0 Months Ended	1 Months Ended	2 Months Ended	3 Months Ended		1 Months Ended		2 Months Ended	3 Months Ended
	Apr. 30, 2013	Dec. 13, 2012	Apr. 30, 2013	Apr. 30, 2012	Jan. 31, 2013	Apr. 30, 2013 Puttable Equity-Linked Senior Notes due 2014 [Member]	Mar. 15, 2013 Series A Preferred Stock [Member]	Mar. 14, 2013 Series A Preferred Stock [Member]	Dec. 13, 2012 Series A Preferred Stock [Member]	Apr. 30, 2013 Series A Preferred Stock [Member]	Apr. 30, 2012 Series A Preferred Stock [Member]	Apr. 30, 2013 Class A common stock [Member]	Mar. 14, 2013 Class A common stock [Member]	Dec. 13, 2012 Class A common stock [Member]	Apr. 30, 2013 Class A common stock [Member]	Mar. 13, 2013 Class A common stock [Member]
Class of Stock [Line Items]
Exchange of preferred stock, aggregate liquidation preference amount									$ 209,447,600
Exchange of preferred stock, preferred shares exchanged									4,188,952
Exchange of preferred stock, common shares issued for preferred stock														13,852,435
Exchange of preferred stock, initial conversion price														$ 15.12
Cash payments related to conversion of preferred stock for Class A common stock		19,069,000
Conversion of stock, shares converted								109,768					362,990
Stock redeemed during period, Shares							101,270			(101,000)
Stock redeemed during period, Value			(5,063,000)				5,100,000			(5,063,000)
Shares received upon settlement of equity call hedge																765,134
Value per share of Class A common stock received upon settlement of equity call hedge																$ 16.82
Class A common stock received upon settlement of equity call hedge																12,868,000
Proceeds from equity call hedge related to Series A preferred stock issuance			10,231,000	0
Preferred Stock (Textual) [Abstract]
Preferred dividends			185,000	3,850,000	13,060,000					185,000	3,850,000
Repayment of debt issuing Class A common stock						138,853,000
Issuance of Class A shares in exchange for Puttable Equity-Linked Senior Notes due 2014, Shares												9,549,721			9,550,000
Cash payment for additional exchange consideration, accrued interest and in lieu of fractional shares	$ 4,860,000