Marketable Securities (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Stockholders' equity		$ 3,183,998	$ 3,243,164	$ 3,245,872	$ 3,300,495
Proceeds from sale of common stock	$ 23,900
Recognized loss on sale of common stock included in interest and other income		5,000
Cumulative Other Comprehensive Loss
Debt Securities, Available-for-sale [Line Items]
Stockholders' equity		(342)	0	0	(95)
Retained Earnings
Debt Securities, Available-for-sale [Line Items]
Stockholders' equity		$ 2,870,974	$ 3,814,948	$ 3,363,654	2,596,259
Accounting Standards Update 2016-01 | Cumulative Other Comprehensive Loss | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale [Line Items]
Stockholders' equity					(1,900)
Accounting Standards Update 2016-01 | Retained Earnings | Cumulative Effect, Period of Adoption, Adjustment
Debt Securities, Available-for-sale [Line Items]
Stockholders' equity					$ 1,900