Investments: - Activity and maturities of available-for-sale securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale securities
Available-for-sale securities, gross unrealized losses that were in effect for less than one year (as a percent)	65.00%
Cost	$ 442,148	$ 425,078
Gains	52,221	38,652
Losses	(8,466)	(13,130)
Fair Value	485,903	450,600
Equity
Available-for-sale securities
Cost	237,317	230,123
Gains	51,054	37,494
Losses	(5,041)	(9,635)
Fair Value	283,330	257,982
Debt
Available-for-sale securities
Cost	201,492	189,700
Gains	1,167	1,158
Losses	(3,423)	(3,491)
Fair Value	199,236	187,367
Cost
Due within one year	8,292	8,559
Due after one year through five years	52,452	50,018
Due after five years through ten years	65,657	56,448
Due after ten years	75,091	74,675
Total	201,492	189,700
Fair Value
Due within one year	8,268	8,612
Due after one year through five years	52,054	49,538
Due after five years through ten years	64,971	55,880
Due after ten years	73,943	73,337
Total	199,236	187,367
Other
Available-for-sale securities
Cost	3,339	5,255
Losses	(2)	(4)
Fair Value	$ 3,337	$ 5,251