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Investments in Marketable Securities - Schedule of Available-For-Sale and Trading Securities (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Available-for-Sale          
Cost $ 66,500,000   $ 66,500,000   $ 104,700,000
Unrealized gains 40,500,000   40,500,000   77,600,000
Unrealized losses (500,000)   (500,000)   (1,800,000)
Fair Value 106,500,000   106,500,000   180,500,000
Trading          
Cost 97,900,000   97,900,000   19,800,000
Unrealized gains     11,500,000   1,900,000
Unrealized losses     (2,700,000)   (2,300,000)
Fair Value 106,700,000   106,700,000   19,400,000
Investments in marketable securities 213,200,000   213,200,000   $ 199,900,000
Realized gains (losses) on available-for-sale securities 6,200,000 $ 700,000 15,400,000 $ 8,700,000  
Realized gains (losses) on trading securities $ 0 $ 0 $ 0 $ 0