Investments: Activity and maturities of available-for-sale securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Investments:
Available-for-sale securities, gross unrealized losses that were in effect for less than one year (as a percent)	27.00%
Available-for-sale securities
Cost	$ 425,078	$ 383,001
Gains	38,652	79,517
Losses	(13,130)	(10,786)
Fair Value	450,600	451,732
Equity
Available-for-sale securities
Cost	230,123	200,892
Gains	37,494	69,536
Losses	(9,635)	(2,163)
Fair Value	257,982	268,265
Debt.
Available-for-sale securities
Cost	189,700	168,182
Gains	1,158	9,981
Losses	(3,491)	(8,619)
Fair Value	187,367	169,544
Cost
Due within one year	8,559	1,744
Due after one year through five years	50,018	48,105
Due after five years through ten years	56,448	67,103
Due after ten years	74,675	51,230
Total	189,700	168,182
Fair Value
Due within one year	8,612	1,790
Due after one year through five years	49,538	47,508
Due after five years through ten years	55,880	67,665
Due after ten years	73,337	52,581
Total	187,367	169,544
Other
Available-for-sale securities
Cost	5,255	13,927
Losses	(4)	(4)
Fair Value	$ 5,251	$ 13,923