Investments: (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Investments:
Available-for-sale securities, gross unrealized losses that were in effect for less than one year (as a percent)	73.00%
Available-for-sale securities
Cost	$ 359,797,000	$ 330,627,000
Gross Unrealized Gains	75,745,000	55,357,000
Gross Unrealized Losses	(10,123,000)	(8,176,000)
Fair Value	425,419,000	377,808,000
Equity
Available-for-sale securities
Cost	182,755,000	153,846,000
Gross Unrealized Gains	68,424,000	45,071,000
Gross Unrealized Losses	(1,053,000)	(3,675,000)
Fair Value	250,126,000	195,242,000
Debt
Available-for-sale securities
Cost	164,941,000	163,127,000
Gross Unrealized Gains	7,319,000	10,286,000
Gross Unrealized Losses	(9,070,000)	(4,501,000)
Fair Value	163,190,000	168,912,000
Cost
Due within one year	1,107,000	2,265,000
Due after one year through five years	46,950,000	43,050,000
Due after five years through ten years	62,860,000	68,893,000
Due after ten years	54,024,000	48,919,000
Total	164,941,000	163,127,000
Fair Value
Due within one year	1,095,000	2,191,000
Due after one year through five years	46,982,000	43,533,000
Due after five years through ten years	61,612,000	71,217,000
Due after ten years	53,501,000	51,971,000
Total	163,190,000	168,912,000
Other
Available-for-sale securities
Cost	12,101,000	13,654,000
Gross Unrealized Gains	2,000
Fair Value	$ 12,103,000	$ 13,654,000