Equities schedule of stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	May 31, 2016	Aug. 31, 2016	Aug. 31, 2014	Mar. 31, 2016	Aug. 30, 2014	Aug. 25, 2014	Sep. 26, 2013
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share		$ 25.00
Stock Redeemed or Called During Period, Price per Share	$ 28.50		$ 29.62
Class B, Series 4 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Redemption Amount		$ 517.5
Proceeds from issuance of preferred stock, net of issuance costs		$ 501.0
Preferred Stock, Shares Outstanding		20,700,000
Preferred Stock, Dividend Rate, Percentage		7.50%
Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Redemption Amount		$ 492.5
Proceeds from issuance of preferred stock, net of issuance costs		$ 476.7
Preferred Stock, Shares Outstanding		19,700,000
Preferred Stock, Dividend Rate, Percentage		6.75%
Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Redemption Amount		$ 420.0
Proceeds from issuance of preferred stock, net of issuance costs		$ 406.2
Preferred Stock, Shares Outstanding		16,800,000
Preferred Stock, Dividend Rate, Percentage		7.10%
Class B, Series 1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Redemption Amount		$ 519.1
Proceeds from issuance of preferred stock, net of issuance costs		$ 549.4
Preferred Stock, Shares Outstanding		20,764,558
Preferred Stock, Dividend Rate, Percentage		7.875%
Preferred Stock, Shares Issued				2,693,195	6,752,188	6,752,188	11,319,175
8% Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Redemption Amount		$ 306.8		$ 67.3
Proceeds from issuance of preferred stock, net of issuance costs		$ 311.2
Preferred Stock, Shares Outstanding		12,272,003
Preferred Stock, Dividend Rate, Percentage		8.00%
London Interbank Offered Rate (LIBOR) [Member] | Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		4.155%
London Interbank Offered Rate (LIBOR) [Member] | Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		4.298%
Maximum | Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		8.00%
Maximum | Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent		8.00%
Capital Equity Certificates
Class of Stock [Line Items]
Stock Redeemed or Called During Period, Value	$ 76.8		$ 200.0