Fair Value Measurements - Summary of the Carrying Amounts and Fair Values of the Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Summary of financial instruments
Bank line of credit	$ 1,017,076	$ 899,718
Senior unsecured notes	6,396,451	7,133,538
Mortgage debt	144,486	623,792
Other debt	94,165	92,385
Carrying Value
Summary of financial instruments
Loans receivable, net	313,326	807,954
Marketable debt securities	18,690	68,630
Bank line of credit	1,017,076	899,718
Term loans	228,288	440,062
Senior unsecured notes	6,396,451	7,133,538
Mortgage debt	144,486	623,792
Other debt	94,165	92,385
Carrying Value \| Interest-rate swap contracts
Summary of financial instruments
Derivative liabilities	2,483	4,857
Carrying Value \| Currency Swap
Summary of financial instruments
Derivative liabilities	10,968	0
Derivative assets	0	2,920
Fair Value \| Level 1
Summary of financial instruments
Senior unsecured notes	6,737,825	7,386,149
Fair Value \| Level 2
Summary of financial instruments
Loans receivable, net	313,242	807,505
Marketable debt securities	18,690	68,630
Bank line of credit	1,017,076	899,718
Term loans	228,288	440,062
Mortgage debt	125,984	609,374
Other debt	94,165	92,385
Fair Value \| Interest-rate swap contracts \| Level 2
Summary of financial instruments
Derivative liabilities	2,483	4,857
Fair Value \| Currency Swap \| Level 2
Summary of financial instruments
Derivative liabilities	10,968	0
Derivative assets	$ 0	$ 2,920

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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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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