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.

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

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

Notes Payable (Unsecured Term Loan) (Details) - USD ($)	6 Months Ended
Notes Payable (Unsecured Term Loan) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unsecured term loan	$ 1,094,420,000	$ 1,098,764,000
Term Loan
Debt Instrument [Line Items]
Unsecured term loan	$ 250,000,000	$ 250,000,000
Term Loan \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.20%
Term Loan \| LIBOR \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.20%
Term Loan \| LIBOR \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.70%
Term Loan \| Federal Funds Rate
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	0.50%
Term Loan \| One-month LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	1.00%
Term Loan \| Base Rate \| Minimum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	0.00%
Term Loan \| Base Rate \| Maximum
Debt Instrument [Line Items]
Basis spread on variable rate (percent)	0.75%