Investment Securities: (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Document Period End Date			Sep. 30, 2015
Investment Maturity Date Range Start	2015
Investment Maturity Date Range End		2036
Investment securities	$ 38,695		$ 38,695	$ 70,952	[1]
Federal tax-exempt debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	3,463		3,463	15,013
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	25,253		25,253	46,209
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	4,039		4,039	0
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	3,687		3,687	3,846
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	$ 2,253		$ 2,253	$ 5,884

[1] December 31, 2014 balances were derived from audited consolidated financial statements.