Marketable Securities (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Cash equivalents and marketable securities
Maximum maturity period of available-for-sale securities (in years)	3 years
Maximum remaining maturity period of available-for-sale securities (in years)	1 year	1 year
Fair Value	$ 390,029	$ 363,542
Amortized Cost	390,170	363,587
Gross Unrealized Gains	159	19
Gross Unrealized Losses	(300)	(64)
Cash, fair value	45,416	44,122
Cash, amortized cost	45,416	44,122
Cash, cash equivalents and marketable securities
Fair Value	435,445	407,664
Amortized Cost	435,586	407,709
Gross Unrealized Gains	159	19
Gross Unrealized Losses	(300)	(64)
Money market funds
Cash equivalents and marketable securities
Fair Value	22,143	10,065
Amortized Cost	22,143	10,065
Gross Unrealized Gains	0	0
Gross Unrealized Losses	$ 0	$ 0
Weighted Rate of Return (as a percent)	0.32%	1.48%
US Government bonds and notes
Cash equivalents and marketable securities
Fair Value	$ 44,247	$ 39,086
Amortized Cost	44,135	39,087
Gross Unrealized Gains	112	0
Gross Unrealized Losses	$ 0	$ (1)
Weighted Rate of Return (as a percent)	1.20%	1.49%
Corporate notes, bonds and commercial paper and other
Cash equivalents and marketable securities
Fair Value	$ 323,639	$ 314,391
Amortized Cost	323,892	314,435
Gross Unrealized Gains	47	19
Gross Unrealized Losses	$ (300)	$ (63)
Weighted Rate of Return (as a percent)	1.61%	1.81%