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Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Fair Value Accounting - Narrative (Details) - USD ($) $ in Millions	Mar. 22, 2011	Sep. 30, 2017
Post-Modification Notes \| Senior Notes
Debt Instrument [Line Items]
Basis spread on variable rate	3.50%
Mortgage securities
Debt Instrument [Line Items]
Aggregate overcollateralization on overcollateralization bonds held		$ 29.9