Debt Debt (Schedule of Unsecured Term Loans) (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Unsecured Debt	$ 400,000
January 2023 [Member]
Unsecured Debt	$ 125,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
November 2023 [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	3.99%
September 2024 [Member]
Unsecured Debt	$ 75,000
Debt Instrument, Interest Rate, Stated Percentage	3.93%
November 2025 [Member]
Unsecured Debt	$ 50,000
Debt Instrument, Interest Rate, Stated Percentage	4.33%
January 2027 [Member]
Unsecured Debt	$ 100,000
Debt Instrument, Interest Rate, Stated Percentage	4.51%

Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: 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

Reference 3: 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(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Name:	us-gaap_UnsecuredDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant