Debt Structure (Detail) - Derivative, Name [Domain] - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Total Outstanding Debt	$ 3,442,244	$ 3,520,000
Debt Instrument, Interest Rate, Effective Percentage	3.60%	3.70%
Contract Maturity	Aug. 12, 2019
Secured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	$ 1,413,793
Debt Instrument, Interest Rate, Effective Percentage	3.40%
Contract Maturity	Nov. 21, 2018
Secured Debt [Member] \| Mortgages [Member]
Debt Instrument [Line Items]
Mortgage Loans on Real Estate	$ 23,500
Secured Debt [Member] \| Mortgages [Member] \| Fixed Rate Debt
Debt Instrument [Line Items]
Mortgage Loans on Real Estate	$ 1,081,373
Debt Instrument, Interest Rate, Effective Percentage	4.00%
Contract Maturity	Jun. 10, 2019
Secured Debt [Member] \| Mortgages [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Mortgage Loans on Real Estate	$ 11,635
Debt Instrument, Interest Rate, Effective Percentage	3.10%
Contract Maturity	Aug. 15, 2015
Secured Debt [Member] \| Fixed Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	$ 1,131,373
Debt Instrument, Interest Rate, Effective Percentage	4.00%
Contract Maturity	May 06, 2019
Secured Debt [Member] \| Variable Rate Debt
Debt Instrument [Line Items]
Total Outstanding Debt	$ 282,420
Debt Instrument, Interest Rate, Effective Percentage	0.80%
Contract Maturity	Jan. 26, 2017
Secured Debt [Member] \| Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	$ 320,800
Secured Debt [Member] \| Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] \| Variable Rate Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	0.70%
Contract Maturity	Feb. 18, 2017
Line of Credit Facility, Amount Outstanding	$ 270,785
Secured Debt [Member] \| Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] \| Fixed Rate Debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.70%
Contract Maturity	Mar. 31, 2017
Line of Credit Facility, Amount Outstanding	$ 50,000
Secured Debt [Member] \| Federal National Mortgage Association Certificates and Obligations (FNMA) [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	0.80%
Line of Credit Facility, Amount Outstanding	$ 270,800
Unsecured Debt [Member]
Debt Instrument [Line Items]
Total Outstanding Debt	$ 2,028,451
Debt Instrument, Interest Rate, Effective Percentage	3.80%
Contract Maturity	Jun. 25, 2020
Unsecured Debt [Member] \| $500 million unsecured revolving credit faciltiy [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	1.28%
Line of Credit Facility, Amount Outstanding	$ 159,000
Unsecured Debt [Member] \| $500 million unsecured revolving credit faciltiy [Member] \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	1.30%
Contract Maturity	Aug. 07, 2017
Line of Credit Facility, Amount Outstanding	$ 159,000
Unsecured Debt [Member] \| Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	3.10%
Contract Maturity	Nov. 10, 2017
Debt Instrument, Face Amount	$ 550,000
Unsecured Debt [Member] \| Senior Notes [Member] \| Fixed Rate Debt
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Effective Percentage	4.50%
Contract Maturity	Dec. 03, 2021
Debt Instrument, Face Amount	$ 1,319,451

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

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=6451184&loc=d3e28551-108399

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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

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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.

+ References

No definition available.

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- Definition

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

+ References

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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 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

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- Definition

Total carrying amount of mortgage loans as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule IV)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 948

-SubTopic 310

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-29.6)

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- Definition

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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