Quarterly Financial Information (Unaudited) - Other-than-temporary Impairment Loss on Held-to-maturity Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2017	Dec. 31, 2017
Other-than-temporary Impairment Loss, Debt Securities, Held-to-maturity, before Tax [Abstract]
Net impairment loss recognized in earnings on held-to-maturity securities		$ 35,418
Bonds B Series [Member]
Other-than-temporary Impairment Loss, Debt Securities, Held-to-maturity, before Tax [Abstract]
Net impairment loss recognized in earnings on held-to-maturity securities	$ 35,400

Amount, before tax, of other-than-temporary impairment (OTTI) on investment in debt security measured at amortized cost (held-to-maturity), recognized in earnings.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=109979856&loc=SL6284422-111562

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