Investment Securities: (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Investment Maturity Date Range Start	2015
Investment Maturity Date Range End	2045
Investment securities	$ 46,254	$ 70,952	[1]
Federal tax-exempt debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	13,760	15,013
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	24,513	46,209
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	4,497	3,846
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	$ 3,484	$ 5,884

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