Resale and Repurchase Agreements Resale and Repurchase Agreements (Narrative) (Details) - Collateral Swap [Member] - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Balance Sheet Offsetting [Line Items]
Collateral Swap Agreements	$ 650.0	$ 650.0
Collateral Already Posted, Aggregate Fair Value	659.1
Collateral Accepted, Aggregate Fair Value	$ 659.8

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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