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Borrowings (Textual) (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |  |
Notes Payable |  | $ 4,500,000,000 | $ 4,499,700,000 |
Average interest rate (percentage) |  | 3.60% |  |
Fixed Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes Payable |  | $ 3,724,800,000 | 3,838,300,000 |
Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Repayments of Debt |  | 156,400,000 |  |
Floating Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes Payable |  | $ 790,000,000 | 650,000,000 |
Variable Rate Debt [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 2.30% |  |
Unsecured Debt |  | $ 300,000,000 |  |
Unsecured Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes Payable |  | $ 3,525,765,000 |  |
Average interest rate (percentage) |  | 3.50% |  |
Unsecured Debt | $1 billion unsecured revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of credit facility, maximum borrowing capacity |  | $ 1,000,000,000 |  |
Line of credit facility, amount outstanding |  | $ 410,000,000 |  |
Unsecured Debt | $1 billion unsecured revolving credit faciltiy [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 2.47% |  |
Letters of credit outstanding amount |  | $ 2,500,000 |  |
Unsecured Debt | Fixed Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes Payable |  | $ 2,292,000,000 | 1,860,000,000 |
Average interest rates (percent) |  | 3.97% |  |
Unsecured Debt | Floating Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Notes Payable | [1] | $ 710,000,000 | 490,000,000 |
Average interest rates (percent) | [1] | 2.41% |  |
Unsecured Debt | KeyBank Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | $ 150,000,000 |  |
Unsecured Debt | Wells Fargo Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | 300,000,000 |  |
Unsecured Debt | Wells Fargo Term Loan 2018 [Domain] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | 250,000,000 |  |
Unsecured Debt | US Bank Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | $ 150,000,000 |  |
Unsecured Debt | Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 3.00% |  |
Debt Instrument, Face Amount |  | $ 550,000,000 |  |
Unsecured Debt | 2011 and 2012 Senior Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | $ 292,000,000 |  |
Unsecured Debt | 3.6% Notes due 2027 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 3.68% |  |
Proceeds from Unsecured Notes Payable |  | $ 593,600,000 |  |
Settlement of Forward Swaps |  | $ 300,000,000 |  |
Interest rate, stated percentage |  | 3.60% |  |
Debt Instrument, Issue Price |  | 99.58% |  |
Debt Instrument, Unamortized Discount, Current |  | $ 2,500,000 |  |
Other Underwriting Expense |  | 3,900,000 |  |
Debt Instrument, Face Amount |  | $ 600,000,000 |  |
Unsecured Debt | Minimum | $1 billion unsecured revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.85% |  |
Unsecured Debt | Minimum | Floating Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate, stated percentage | [1] | 2.31% |  |
Unsecured Debt | Minimum | KeyBank Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.90% |  |
Unsecured Debt | Minimum | Wells Fargo Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.90% |  |
Unsecured Debt | Minimum | US Bank Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.90% |  |
Unsecured Debt | Minimum | Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 0.90% |  |
Unsecured Debt | Maximum | $1 billion unsecured revolving credit facility [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of credit facility, maximum borrowing capacity |  | $ 1,500,000,000 |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.55% |  |
Unsecured Debt | Maximum | Floating Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Interest rate, stated percentage | [1] | 2.47% |  |
Unsecured Debt | Maximum | KeyBank Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.75% |  |
Unsecured Debt | Maximum | Wells Fargo Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.90% |  |
Unsecured Debt | Maximum | US Bank Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.90% |  |
Unsecured Debt | Maximum | Term Loan [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  | 1.75% |  |
Unsecured Debt | Variable Rate Debt [Member] | $1 billion unsecured revolving credit faciltiy [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 2.50% |  |
Line of credit facility, amount outstanding |  | $ 410,000,000 |  |
Unsecured Debt | Fixed Rate Debt |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Term |  | 6 years 10 months 15 days |  |
Unsecured Debt | Fixed Rate Debt | Senior Notes |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 4.00% |  |
Debt Instrument, Face Amount |  | $ 2,292,000,000 |  |
Unsecured Debt | Fixed Rate Debt | Public Income Notes [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | 2,000,000,000 |  |
Unsecured Debt | Fixed Rate Debt | Private Placement [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Face Amount |  | 0 |  |
Debt Instrument, Decrease, Forgiveness |  | $ 18,000,000 |  |
Unsecured Debt | Fixed Rate Debt | Minimum |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Term |  | 7 years |  |
Unsecured Debt | Fixed Rate Debt | Maximum |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Debt Instrument, Term |  | 12 years |  |
Secured Debt [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 3.80% |  |
Secured Debt [Member] | FNMA |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of credit facility, maximum borrowing capacity |  | $ 80,000,000 |  |
Line of credit facility, amount outstanding |  | 80,000,000 |  |
Line of Credit Facility, Current Borrowing Capacity |  | 80,000,000 |  |
Secured Debt [Member] | Amortization Other [Member] | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Mortgage Loans on Real Estate |  | $ 12,000,000 |  |
Secured Debt [Member] | Variable Rate Debt [Member] | FNMA |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 1.80% |  |
Secured Debt [Member] | Fixed Rate Debt | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Average interest rate (percentage) |  | 4.00% |  |
Mortgage Loans on Real Estate |  | $ 882,752,000 |  |
Parent Company [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Unsecured Debt |  | 3,525,765,000 | 3,180,624,000 |
Proceeds from notes payable |  | 597,480,000 | $ 300,000,000 | $ 395,960,000
Grand Cypress [Member] | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Mortgage Loans on Real Estate |  | 15,800,000 |  |
Venue at Stonebridge Ranch [Member] | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Mortgage Loans on Real Estate |  | 13,900,000 |  |
La Valencia At Starwood | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Mortgage Loans on Real Estate |  | 20,100,000 |  |
Colonial Grand at Trinity Commons [Member] | Mortgages |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Mortgage Loans on Real Estate |  | 27 |  |
Line of Credit Facility, Draws [Member] | Secured Debt [Member] | FNMA |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of credit facility, amount outstanding |  | 805,000,000 |  |
Line of Credit [Member] | Secured Debt [Member] | FNMA |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Line of credit facility, amount outstanding |  | $ 885,000,000 |  |
[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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