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Marketable Investment Securities - Additional information (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Marketable Investment Securities        
Proceeds from Sale and Maturity of Marketable Securities $ 8,500,000 $ 10,400,000 $ 17,500,000 $ 27,000,000
Available-for-sale Securities, Gross Realized Gain (Loss) 0 0 0 0
Other-than-temporary impairments $ 0 $ 0 $ 0 $ 0