Investment Securities - Narrative (Details) $ in Millions	Sep. 30, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Marketable Securities [Abstract]
Number of securities in unrealized loss positions for less than 12 months	14	34
Number of securities in unrealized loss position greater than 12 months	13	4
Securities pledged as collateral \| $	$ 50.4	$ 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