Investment Securities: (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Investment Maturity Date Range Start	2014
Investment Maturity Date Range End	2045
Investment securities	$ 79,896	$ 70,869	[1]
Federal tax-exempt debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	18,524	21,968
Corporate Bond Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	51,584	45,696
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	4,228	0
Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Investment securities	$ 5,560	$ 3,205

[1] December 31, 2013 balances were derived from audited consolidated financial statements.