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Fair Value of Financial Instruments (Narrative) (Details) - USD ($) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Non-Recurring Basis [Abstract] |  |  |
Transfers in or out of level 3 | $ 0 | $ 0 |
Changes in unrealized gains and losses | 0 | $ 0 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Available-for-sale Securities | 657,460,000 |  | $ 1,074,169,000
Fair Value, Measurements, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Available-for-sale Securities | 657,460,000 |  | 1,074,169,000
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring [Member] |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Available-for-sale Securities | $ 0 |  | $ 0

X
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy. + References No definition available. + Details
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