Fair Values of Financial Instruments - Transfer Between Levels 1, 2 and 3 and Reasons for Transfers (Details) - USD ($)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Transfer between in or out of Level 3	$ 0	$ 0

Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

No definition available.

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration