Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Summary of financial instruments
Bank line of credit	$ 397,432	$ 838,516
Senior unsecured notes	9,120,107	7,589,960
Mortgage debt	932,212	982,785
Other debt	94,445	97,022
Carrying Value
Summary of financial instruments
Loans receivable, net	768,743	906,961
Marketable debt securities	102,958	231,442
Marketable equity securities	39	43
Bank line of credit	397,432	838,516
Term loan	524,807	212,986
Senior unsecured notes	9,120,107	7,589,960
Mortgage debt	932,212	982,785
Other debt	94,445	97,022
Carrying Value \| Warrants
Summary of financial instruments
Derivative assets	55	2,220
Carrying Value \| Interest-rate swap contracts
Summary of financial instruments
Derivative assets	196	178
Derivative liabilities	6,251	7,663
Carrying Value \| Currency Swap
Summary of financial instruments
Derivative assets	1,551	929
Fair Value \| Level 1
Summary of financial instruments
Marketable debt securities	102,958	252,125
Marketable equity securities	39	43
Senior unsecured notes	9,390,668	8,187,458
Fair Value \| Level 2
Summary of financial instruments
Loans receivable, net	770,052	898,522
Bank line of credit	397,432	838,516
Term loan	524,807	212,986
Mortgage debt	963,786	1,025,091
Other debt	94,445	97,022
Fair Value \| Warrants \| Level 3
Summary of financial instruments
Derivative assets	55	2,220
Fair Value \| Interest-rate swap contracts \| Level 2
Summary of financial instruments
Derivative assets	196	178
Derivative liabilities	6,251	7,663
Fair Value \| Currency Swap \| Level 2
Summary of financial instruments
Derivative assets	$ 1,551	$ 929

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Fair value of investments in debt securities classified as held-to-maturity.

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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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Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

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Carrying value as of the balance sheet date of loans from a bank with maturities initially due after one year or beyond the operating cycle if longer, excluding current portion.

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The carrying amount as of the balance sheet date for the aggregate of other miscellaneous borrowings owed by the reporting entity.

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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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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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