Investment Securities: (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Investment Maturity Date Range Start	2015
Investment Maturity Date Range End	2037
Investment securities	$ 36,223	$ 70,952	[1]
Federal tax-exempt debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	10,999	15,013
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	18,003	46,209
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	3,986	3,846
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	$ 3,235	$ 5,884

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