Investments in Equity Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Equity securities	$ 496,034	$ 576,647
Fair Value
Schedule of Available-for-sale Securities [Line Items]
Cost	219,597	206,005
Gross Unrealized, Gains	285,629	375,199
Gross Unrealized, Losses	(9,192)	(4,557)
Equity securities	496,034	576,647
Fair Value | Common stock
Schedule of Available-for-sale Securities [Line Items]
Cost	95,447	81,855
Gross Unrealized, Gains	232,206	272,309
Gross Unrealized, Losses	(7,218)	(1,960)
Equity securities	320,435	352,204
Fair Value | Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	124,150	124,150
Gross Unrealized, Gains	53,423	102,890
Gross Unrealized, Losses	(1,974)	(2,597)
Equity securities	175,599	224,443
Carrying Value
Schedule of Available-for-sale Securities [Line Items]
Equity securities	496,034	576,647
Carrying Value | Common stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities	320,435	352,204
Carrying Value | Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Equity securities	$ 175,599	$ 224,443