EQUITY - Preferred Stock Issuance, Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended	9 Months Ended
	Mar. 19, 2018	Mar. 07, 2017	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 05, 2019
Class of Stock [Line Items]
Issuance of preferred shares, net of issuance costs	$ 289.8	$ 264.4
Series A Preferred Share
Class of Stock [Line Items]
Preferred stock, shares issued		11,000,000
Dividend rate per annum		6.375%
Issuance of preferred shares, net of issuance costs		$ 275.0
Series B Preferred Share
Class of Stock [Line Items]
Preferred stock, shares issued	12,000,000
Dividend rate per annum	6.375%
Issuance of preferred shares, net of issuance costs	$ 300.0
Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued			11,000,000	11,000,000	11,000,000
Preferred stock, shares outstanding			11,000,000	11,000,000	11,000,000
Dividends declared per share (in USD per share)			$ 0.398438
Series A Preferred Stock | AGM LLC
Class of Stock [Line Items]
Preferred stock, shares issued						1
Preferred stock, liquidation preference (in USD per share)						$ 25.00
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued			12,000,000	12,000,000	12,000,000
Preferred stock, shares outstanding			12,000,000	12,000,000	12,000,000
Series B Preferred Stock | Equity, Redemption, Period One
Class of Stock [Line Items]
Redemption price (in USD per share)			$ 25.50	$ 25.50
Required days notice				30 days
Number of days within occurrence				60 days
Series B Preferred Stock | AGM LLC
Class of Stock [Line Items]
Preferred stock, shares issued						1
Preferred stock, liquidation preference (in USD per share)						$ 25.00
Preferred Stock
Class of Stock [Line Items]
Increase to distribution rate (as a percent)				5.00%
Preferred Stock | Equity, Redemption, Period Two
Class of Stock [Line Items]
Redemption price (in USD per share)			25.00	$ 25.00
Preferred Stock | Equity, Redemption, Period One
Class of Stock [Line Items]
Redemption price (in USD per share)			$ 25.25	$ 25.25
Required days notice				30 days
Number of days within occurrence				60 days