Debt Obligations - Consolidated Debt Obligations Outstanding (Parenthetical) (Details) - Unsecured Debt [Member] - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,789,110	$ 1,721,110
$600 million Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Unsecured credit facility, maximum borrowing capacity	600,000	600,000
Long-term Debt, Gross	160,500	92,500
$350M 3.95% Guaranteed Notes due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 350,000	$ 350,000
Debt instrument interest rate	3.95%	3.95%
Debt instrument maturity year	2023	2023
$250.0M 4.10% Guaranteed Notes due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 250,000	$ 250,000
Debt instrument interest rate	4.10%	4.10%
Debt instrument maturity year	2024	2024
$450.0M 3.95% Guaranteed Notes due 2027 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 450,000	$ 450,000
Debt instrument interest rate	3.95%	3.95%
Debt instrument maturity year	2027	2027
$250M 4.55% Guaranteed Notes due 2029 [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 250,000	$ 250,000
Debt instrument interest rate	4.55%	4.55%
Debt instrument maturity year	2029	2029

Debt instrument maturity year.

No definition available.

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

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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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Period Type:	duration

- 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://fasb.org/us-gaap/role/ref/legacyRef
-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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- Details

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- Details

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- Details

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- Details

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