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Cash, Cash Equivalents, Marketable Securities and Restricted Cash - Additional Information (Details) - USD ($)
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Jun. 30, 2018
Dec. 31, 2017
Cash, Cash Equivalents, and Short-term Investments [Abstract]        
Realized gain (loss) on debt securities $ 0 $ 0    
Fair value of available-for-sale debt securities in unrealized loss positions 38,500,000 403,100,000    
Cash and cash equivalents 220,578,000 79,749,000 $ 73,311,000 $ 49,060,000
Restricted cash 2,610,000 2,626,000 2,130,000 2,177,000
Cash, cash equivalents, and restricted cash shown in the Consolidated Statements of Cash Flows $ 223,188,000 $ 82,375,000 $ 75,441,000 $ 51,237,000