Investments (Schedule of Unrealized Appreciation) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2017	Mar. 31, 2016
Investments [Abstract]
Net change in unrealized appreciation on available-for-sale securities before deferred tax	$ 522	$ 343
Deferred income tax	(178)	(117)
Net change in unrealized appreciation on available-for-sale securities	$ 344	$ 226

The change in deferred tax associated with the change in unrealized holding gain (loss) on available-for-sale securities included in a separate component of shareholders' equity during the period.

No definition available.

+ Details

Name:	nsec_ChangeInDeferredTaxUnrealizedAppreciation
Namespace Prefix:	nsec_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

This item represents the pretax change in net unrealized holding gain (loss) on available-for-sale securities that has been included in accumulated other comprehensive income, a separate component of shareholders' equity, during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27357-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The change in unrealized holding gain (loss), net of tax, on available-for-sale securities included in a separate component of shareholders' equity during the period.

+ References

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLossNetOfTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

Name:	us-gaap_InvestmentsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration