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Debt Commitments - Unsecured Revolving Credit Facility (Details) - Credit Facility [Member] | 12 Months Ended
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Dec. 31, 2017 USD ($)
Debt Instrument [Line Items] |
Weighted per annum interest rate on outstanding line of credit | 2.50%
Minimum [Member] |
Debt Instrument [Line Items] |
Percentage fee paid for unused portion of credit facility | 0.10%
Maximum [Member] |
Debt Instrument [Line Items] |
Percentage fee paid for unused portion of credit facility | 0.125%
LIBOR [Member] |
Debt Instrument [Line Items] |
Per annum interest rate over LIBOR | 0.95%
Unsecured Revolving Credit Facility [Member] |
Debt Instrument [Line Items] |
Credit facility, maximum borrowing capacity | $ 700,000,000
Letter of Credit Subfacility [Member] |
Debt Instrument [Line Items] |
Credit facility, maximum borrowing capacity | $ 55,000,000

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Balance Type: | credit
Period Type: | instant

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- Definition The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility. + References No definition available. + Details
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- Details
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- Details
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- Details
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