Nature of Operations and Summary of Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents	$ 215,723	$ 87,206	$ 328,384	$ 181,792
Amortized Cost	610,185	432,966
Fair market value	610,185	432,966
Total Assets Measured at Fair Value - Amortized cost	825,908	520,172
Total cash and cash equivalents and available for sale securities, fair market value	$ 825,908	520,172
Marketable Securities
Maximum period for redemption	1 year
Bonds and bond funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 117,928	267,243
Fair market value	117,928	267,243
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	492,257	165,723
Fair market value	$ 492,257	$ 165,723