Investments in Equity Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 208,785	$ 220,587
Gross Unrealized, Gains	410,444	453,298
Gross Unrealized, Losses	(5,204)	(4,685)
Carrying and fair value of equity securities	614,025	669,200
Common stock
Schedule of Available-for-sale Securities [Line Items]
Cost	83,709	94,998
Gross Unrealized, Gains	339,297	351,906
Gross Unrealized, Losses	(3,486)	(1,046)
Carrying and fair value of equity securities	419,520	445,858
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	125,076	125,589
Gross Unrealized, Gains	71,147	101,392
Gross Unrealized, Losses	(1,718)	(3,639)
Carrying and fair value of equity securities	$ 194,505	$ 223,342