Borrowings (Indebtedness) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Total outstanding debt	$ 4,530,000	$ 4,502,057
Average interest rate (percentage)	3.80%
Secured Debt [Member]
Debt Instrument [Line Items]
Average interest rate (percentage)	4.60%
Unsecured Debt
Debt Instrument [Line Items]
Total outstanding debt	$ 4,053,302	3,525,765
Average interest rate (percentage)	3.70%
Mortgages
Debt Instrument [Line Items]
Average interest rate (percentage)	4.44%
$1 billion unsecured revolving credit faciltiy [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Average interest rate (percentage)	3.42%
Wells Fargo Term Loan [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 300,000	550,000
Average interest rate (percentage)	2.30%
Contract maturity	Mar. 01, 2022
Variable Rate Debt [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Average interest rate (percentage)	3.30%
Contract maturity	Jan. 22, 2020
Unsecured Debt	$ 600,000	300,000
$1 billion unsecured revolving credit facility [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	540,000
Secured Debt [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Total outstanding debt	475,026	976,292
Variable Rate Credit Facility [Member] \| FNMA \| Secured Debt [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 0	80,000
Contract maturity
Fixed Rate Debt \| Mortgages \| Secured Debt [Member]
Debt Instrument [Line Items]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate	$ 476,161	882,752
Average interest rate (percentage)	4.60%
Contract maturity	Feb. 23, 2031
Fixed Rate Debt \| Senior Notes \| Unsecured Debt
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 2,642,000	2,292,000
Average interest rate (percentage)	4.00%
Contract maturity	Jul. 12, 2025
Variable Rate Debt [Member] \| $1 billion unsecured revolving credit faciltiy [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 540,000	410,000
Average interest rate (percentage)	3.40%
Contract maturity	Apr. 15, 2020
Fair Market Value Adjustment and Debt Issuance Cost [Member] \| Secured Debt [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ (1,135)	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	$ (28,698)	$ (26,235)
Minimum \| $1 billion unsecured revolving credit facility [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.85%
Maximum \| $1 billion unsecured revolving credit facility [Member] \| Unsecured Debt
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.55%

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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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.

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Amount of investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate.

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Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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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).

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