Summary of Investments (Narrative) (Details) - USD ($)	3 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Investments [Abstract]
Sale of held-to-maturity securities	$ 0	$ 0
Trading securities	14,705,000		$ 16,842,000
Remaining potential contractual obligation	$ 15,380,000

Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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