Investment Securities - Narrative (Details) $ in Millions	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Marketable Securities [Abstract]
Securities with unrealized losses	24	34
Positions in unrealized loss for greater than one year	17	4
Pledged securities \| $	$ 51.6	$ 50.9

Number of investment positions in available-for-sale investments that have been in a continuous unrealized loss position for greater than or equal to a year for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

No definition available.

Name:	us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositionsGreaterThanOrEqualToOneYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant