Investment Securities - Narrative (Details) $ in Millions	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Marketable Securities [Abstract]
Number of securities in unrealized loss positions for less than 12 months	26	24
Number of securities in unrealized loss position greater than 12 months	17	17
Securities pledged as collateral \| $	$ 51.9	$ 51.6

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