Nature of Operations and Summary of Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents	$ 87,206	$ 328,384	$ 181,792	$ 146,766
Amortized Cost	432,966	360,988
Gross unrealized holding losses		(43)
Fair market value	432,966	360,945
Total Assets Measured at Fair Value - Amortized cost	520,172	689,372
Total cash and cash equivalents and available for sale securities, fair market value	$ 520,172	689,329
Marketable Securities
Maximum period for redemption	1 year
Bonds and bond funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 267,243	229,825
Gross unrealized holding losses		(42)
Fair market value	267,243	229,783
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	165,723	131,163
Gross unrealized holding losses		(1)
Fair market value	$ 165,723	$ 131,162