Fair Value Accounting - Narrative (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($) Rate
Debt Instrument [Line Items]
Debt securities	$ 18.4
Post-Modification Notes \| Senior Notes
Debt Instrument [Line Items]
Basis spread on variable rate \| Rate	3.50%
Mortgage securities
Debt Instrument [Line Items]
Aggregate overcollateralization on overcollateralization bonds held	$ 27.8

Amount of investment in debt security measured at amortized cost (held-to-maturity), investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

No definition available.

Name:	us-gaap_DebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant