Nature of Operations and Summary of Significant Accounting Policies - Marketable Securities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total cash and cash equivalents	$ 328,384	$ 181,792	$ 146,766	$ 203,035
Amortized Cost	360,988	503,731
Gross unrealized holding losses	(43)	(228)
Marketable securities	360,945	503,503
Total Assets Measured at Fair Value - Amortized cost	689,372	685,523
Total cash and cash equivalents and available for sale securities, fair market value	$ 689,329	685,295
Marketable Securities
Maximum period for redemption	1 year
Bonds and bond funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 229,825	344,479
Gross unrealized holding losses	(42)	(228)
Marketable securities	229,783	344,251
Commercial paper
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	131,163	159,252
Gross unrealized holding losses	(1)
Marketable securities	$ 131,162	$ 159,252