Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ 1,530,241	$ (551,365)	$ (334,810)
Other comprehensive loss	3,124	(163)	(754)
Balance at end of period	1,174,810	1,530,241	(551,365)
Unrealized Gain (Loss) on Available-for-Sale Debt Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1,320)	(1,157)
Other comprehensive loss	3,124	(163)
Balance at end of period	$ 1,804	$ (1,320)	$ (1,157)