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Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Loss (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance at beginning of period $ 1,174,810 $ 1,530,241 $ (551,365)
Other comprehensive income 1,200 3,124 (163)
Balance at end of period 2,561,375 1,174,810 1,530,241
Unrealized Gain on Available-for-Sale Debt Securities      
AOCI Attributable to Parent, Net of Tax [Roll Forward]      
Balance at beginning of period 1,804 (1,320)  
Other comprehensive income 1,200 3,124  
Balance at end of period $ 3,004 $ 1,804 $ (1,320)