Investments in Equity Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 206,268	$ 220,587
Gross Unrealized, Gains	413,314	453,298
Gross Unrealized, Losses	(6,775)	(4,685)
Carrying and fair value of equity securities	612,807	669,200
Common stock
Schedule of Available-for-sale Securities [Line Items]
Cost	87,185	94,998
Gross Unrealized, Gains	366,106	351,906
Gross Unrealized, Losses	(5,691)	(1,046)
Carrying and fair value of equity securities	447,600	445,858
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	119,083	125,589
Gross Unrealized, Gains	47,208	101,392
Gross Unrealized, Losses	(1,084)	(3,639)
Carrying and fair value of equity securities	$ 165,207	$ 223,342