Cash and Available-for-Sale Securities - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Interest income from available-for-sale securities	$ 4.4	$ 4.8

Amount before accretion (amortization) of purchase discount (premium) of interest income on nonoperating securities.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.7(b))
-URI http://asc.fasb.org/extlink&oid=116634182&loc=SL114868664-224227

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Name:	us-gaap_InvestmentIncomeInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration