Investments: (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Investments:
Available-for-sale securities, gross unrealized losses that were in effect for less than one year (as a percent)	95.00%
Available-for-sale securities
Cost	$ 330,627,000	$ 325,127,000
Gross Unrealized Gains	55,357,000	48,845,000
Gross Unrealized Losses	(8,176,000)	(22,600,000)
Fair Value	377,808,000	351,372,000
Equity
Available-for-sale securities
Cost	153,846,000	149,263,000
Gross Unrealized Gains	45,071,000	29,789,000
Gross Unrealized Losses	(3,675,000)	(9,996,000)
Fair Value	195,242,000	169,056,000
Debt
Available-for-sale securities
Cost	163,127,000	160,218,000
Gross Unrealized Gains	10,286,000	18,021,000
Gross Unrealized Losses	(4,501,000)	(11,063,000)
Fair Value	168,912,000	167,176,000
Cost
Due within one year	2,265,000	2,074,000
Due after one year through five years	43,050,000	32,149,000
Due after five years through ten years	68,893,000	68,987,000
Due after ten years	48,919,000	57,008,000
Total	163,127,000	160,218,000
Fair Value
Due within one year	2,191,000	2,051,000
Due after one year through five years	43,533,000	32,598,000
Due after five years through ten years	71,217,000	74,010,000
Due after ten years	51,971,000	58,517,000
Total	168,912,000	167,176,000
Other
Available-for-sale securities
Cost	13,654,000	15,646,000
Gross Unrealized Gains		1,035,000
Gross Unrealized Losses		(1,541,000)
Fair Value	$ 13,654,000	$ 15,140,000