Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk Financial Instruments with Off-Balance Sheet Risk and Concentrations of Credit Risk - TBAs and Forward Settling Securities (Details) - USD ($)	Dec. 31, 2018	Sep. 30, 2018
Derivative Asset, Fair Value, Gross Asset	$ 5,848.8	$ 8,035.9
Derivative Liability, Fair Value, Gross Liability	6,276.9	8,511
TBA securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	3,700,000	1,200,000
Derivative Liability, Notional Amount	(5,200,000)	(293,200,000)
Derivative Liability, Fair Value, Gross Liability	0	(600,000)
Derivative Asset, Notional Amount	718,400,000	721,500,000
TBA securities sold [Member]
Derivative Asset, Fair Value, Gross Asset	0	3,200,000
Derivative Liability, Notional Amount	(1,651,600,000)	0
Derivative Liability, Fair Value, Gross Liability	(11,700,000)	0
Derivative Asset, Notional Amount	8,300,000	1,099,500,000
Forward settling securities purchased [Member]
Derivative Asset, Fair Value, Gross Asset	2,800,000	500,000
Derivative Asset, Notional Amount	505,800,000	614,300,000
Over the Counter (OTC) Commodity Contracts [Member]
Derivative Asset, Fair Value, Gross Asset		100,000
Derivative Liability, Notional Amount	(261,800,000)	0
Derivative Liability, Fair Value, Gross Liability	$ (1,000,000)	0
Derivative Asset, Notional Amount		$ 427,200,000