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Borrowings (Indebtedness) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 4,500,000 | $ 4,499,700
Average interest rate (percentage) |  | 3.60% |
Contract maturity |  | Mar. 11, 2022 |
Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Average interest rate (percentage) |  | 3.80% |
Contract maturity |  | Sep. 13, 2019 |
Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 3,525,765 |
Average interest rate (percentage) |  | 3.50% |
Contract maturity |  | Dec. 19, 2022 |
Mortgages |  |  |
Debt Instrument [Line Items] |  |  |
Repayments of Debt |  | $ 156,400 |
FNMA | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Line of credit facility, amount outstanding |  | $ 80,000 |
$1 billion unsecured revolving credit faciltiy [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Average interest rate (percentage) |  | 2.47% |
Term Loan [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Face Amount |  | $ 550,000 |
Average interest rate (percentage) |  | 3.00% |
Contract maturity |  | Apr. 17, 2018 |
Fixed Rate Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 3,724,800 | 3,838,300
Fixed Rate Debt | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | 2,292,000 | 1,860,000
Variable Rate Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | 790,000 | 650,000
Variable Rate Debt | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt | [1] | 710,000 | 490,000
Fair Market Value Adjustment and Debt Issuance Cost [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | (12,700) | $ 11,400
$1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Line of credit facility, amount outstanding |  | 410,000 |
Secured Debt [Member] | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | 976,292 |
Variable Rate Credit Facility [Member] | FNMA | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Line of credit facility, amount outstanding |  | $ 80,000 |
Average interest rate (percentage) |  | 1.80% |
Contract maturity |  | Dec. 01, 2018 |
Fixed Rate Debt | Mortgages | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Mortgage Loans on Real Estate |  | $ 882,752 |
Average interest rate (percentage) |  | 4.00% |
Contract maturity |  | Oct. 09, 2019 |
Fixed Rate Debt | Senior Notes | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Face Amount |  | $ 2,292,000 |
Average interest rate (percentage) |  | 4.00% |
Contract maturity |  | Nov. 13, 2024 |
Freddie Mac | FNMA | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Average interest rate (percentage) |  | 1.80% |
Freddie Mac | $1 billion unsecured revolving credit faciltiy [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Line of credit facility, amount outstanding |  | $ 410,000 |
Average interest rate (percentage) |  | 2.50% |
Contract maturity |  | Apr. 15, 2020 |
Fair Market Value Adjustment and Debt Issuance Cost [Member] | $1 billion unsecured revolving credit facility [Member] | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Line of credit facility, amount outstanding |  | $ 13,540 |
Fair Market Value Adjustment and Debt Issuance Cost [Member] | $1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Face Amount |  | $ (26,235) |
Minimum | Term Loan [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.90% |
Minimum | $1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.85% |
Maximum | Term Loan [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.75% |
Maximum | $1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.55% |
[1] | Amounts are adjusted to reflect interest rate swap and cap agreements in effect at December 31, 2017, and 2016, respectively, which results in paying fixed interest payments over the terms of the interest rate swaps and on changes in interest rates above the strike rate of the cap.

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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 The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + 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)) -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 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://www.xbrl.org/2003/role/presentationRef -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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