Equities - Summary of Outstanding Preferred Stock (Details) $ / shares in Units, $ in Millions	12 Months Ended
	Aug. 31, 2018 USD ($) $ / shares shares
Class of Stock [Line Items]
Preferred Stock, Liquidation Preference Per Share | $ / shares	$ 25.00
8% Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Outstanding | shares	12,272,003
Preferred Stock, Redemption Amount	$ 306.8
Proceeds from issuance of preferred stock, net of issuance costs	$ 311.2
Preferred Stock, Dividend Rate, Percentage	8.00%
Class B, Series 1 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Outstanding | shares	21,459,066
Preferred Stock, Redemption Amount	$ 536.5
Proceeds from issuance of preferred stock, net of issuance costs	$ 569.3
Preferred Stock, Dividend Rate, Percentage	7.875%
Class B, Series 2 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Outstanding | shares	16,800,000
Preferred Stock, Redemption Amount	$ 420.0
Proceeds from issuance of preferred stock, net of issuance costs	$ 406.2
Preferred Stock, Dividend Rate, Percentage	7.10%
Class B, Series 2 Preferred Stock [Member] | Maximum
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent	8.00%
Class B, Series 2 Preferred Stock [Member] | London Interbank Offered Rate (LIBOR) [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent	4.298%
Class B, Series 3 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Outstanding | shares	19,700,000
Preferred Stock, Redemption Amount	$ 492.5
Proceeds from issuance of preferred stock, net of issuance costs	$ 476.7
Preferred Stock, Dividend Rate, Percentage	6.75%
Class B, Series 3 Preferred Stock [Member] | Maximum
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent	8.00%
Class B, Series 3 Preferred Stock [Member] | London Interbank Offered Rate (LIBOR) [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Dividends, Percent	4.155%
Class B, Series 4 Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Outstanding | shares	20,700,000
Preferred Stock, Redemption Amount	$ 517.5
Proceeds from issuance of preferred stock, net of issuance costs	$ 501.0
Preferred Stock, Dividend Rate, Percentage	7.50%