Investments in Equity Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Equity securities	$ 472,389	$ 576,647
Fair Value
Schedule of Available-for-sale Securities [Line Items]
Cost	250,603	206,005
Gross Unrealized, Gains	232,120	375,199
Gross Unrealized, Losses	(10,334)	(4,557)
Equity securities	472,389	576,647
Fair Value | Common stock
Schedule of Available-for-sale Securities [Line Items]
Cost	126,453	81,855
Gross Unrealized, Gains	170,313	272,309
Gross Unrealized, Losses	(8,739)	(1,960)
Equity securities	288,027	352,204
Fair Value | Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	124,150	124,150
Gross Unrealized, Gains	61,807	102,890
Gross Unrealized, Losses	(1,595)	(2,597)
Equity securities	184,362	224,443
Carrying Value
Schedule of Available-for-sale Securities [Line Items]
Equity securities	472,389	576,647
Carrying Value | Common stock
Schedule of Available-for-sale Securities [Line Items]
Equity securities	288,027	352,204
Carrying Value | Preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Equity securities	$ 184,362	$ 224,443