Investments in Equity Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 211,440	$ 220,587
Gross Unrealized, Gains	404,192	453,298
Gross Unrealized, Losses	(4,254)	(4,685)
Carrying and fair value of equity securities	611,378	669,200
Common stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	90,080	94,998
Gross Unrealized, Gains	333,397	351,906
Gross Unrealized, Losses	(2,148)	(1,046)
Carrying and fair value of equity securities	421,329	445,858
Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	121,360	125,589
Gross Unrealized, Gains	70,795	101,392
Gross Unrealized, Losses	(2,106)	(3,639)
Carrying and fair value of equity securities	$ 190,049	$ 223,342