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Costs incurred during the period, such as those relating to general administration and policy maintenance that do not vary with and are not primarily related to the acquisition or renewal of insurance contracts.

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Borrowings (Textual) (Details) - USD ($) $ in Thousands	12 Months Ended
Borrowings (Textual) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Notes Payable	$ 4,530,000	$ 4,502,057
Average interest rate (percentage)	3.80%
Mortgages
Debt Instrument [Line Items]
Average interest rate (percentage)	4.44%
Proceeds from Issuance of Secured Debt	$ 172,000
Debt Instrument, Periodic Payment, Principal	10,600
Unsecured Debt
Debt Instrument [Line Items]
Notes Payable	$ 4,053,302	3,525,765
Average interest rate (percentage)	3.70%
Unsecured Debt \| $1 billion unsecured revolving credit facility [Member]
Debt Instrument [Line Items]
Line of credit facility, amount outstanding	$ 540,000
Line of Credit Facility, Current Borrowing Capacity	$ 1,000,000
Unsecured Debt \| $1 billion unsecured revolving credit faciltiy [Member]
Debt Instrument [Line Items]
Average interest rate (percentage)	3.42%
Letters of credit outstanding amount	$ 4,200
Unsecured Debt \| Variable Rate Debt [Member]
Debt Instrument [Line Items]
Average interest rate (percentage)	3.30%
Unsecured Debt	$ 600,000	300,000
Unsecured Debt \| KeyBank Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 150,000
Unsecured Debt \| Wells Fargo Term Loan 2022 [Member]
Debt Instrument [Line Items]
Interest rate, stated percentage	2.32%
Debt Instrument, Face Amount	$ 300,000
Unsecured Debt \| Wells Fargo Term Loan 2018 [Domain]
Debt Instrument [Line Items]
Repayments of Unsecured Debt	250,000
Unsecured Debt \| US Bank Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	150,000
Unsecured Debt \| Private Placement [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 242,000
Unsecured Debt \| 4.2% Notes due 2028 [Member]
Debt Instrument [Line Items]
Average interest rate (percentage)	4.21%
Proceeds from Unsecured Notes Payable	$ 395,000
Settlement of Forward Swaps	$ 200,000
Interest rate, stated percentage	4.20%
Debt Instrument, Issue Price	99.403%
Debt Instrument, Unamortized Discount, Current	$ 2,400
Other Underwriting Expense	2,600
Debt Instrument, Face Amount	$ 400,000
Debt Instrument, Term	10 years
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 \| KeyBank Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.90%
Unsecured Debt \| Minimum \| Wells Fargo Term Loan 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.90%
Unsecured Debt \| Minimum \| Wells Fargo Term Loan 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.75%
Unsecured Debt \| Minimum \| US Bank 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
Debt Instrument, Basis Spread on Variable Rate	1.55%
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 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.75%
Unsecured Debt \| Maximum \| Wells Fargo Term Loan 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.65%
Unsecured Debt \| Maximum \| US Bank Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.90%
Unsecured Debt \| Variable Rate Debt [Member] \| $1 billion unsecured revolving credit faciltiy [Member]
Debt Instrument [Line Items]
Average interest rate (percentage)	3.40%
Line of credit facility, amount outstanding	$ 540,000	410,000
Unsecured Debt \| Fixed Rate Debt
Debt Instrument [Line Items]
Debt Instrument, Term	6 years 5 months 15 days
Unsecured Debt \| Fixed Rate Debt \| Senior Notes
Debt Instrument [Line Items]
Average interest rate (percentage)	4.00%
Debt Instrument, Face Amount	$ 2,642,000	2,292,000
Unsecured Debt \| Fixed Rate Debt \| Public Income Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	2,400,000
Unsecured Debt \| Fixed Rate Debt \| Private Placement [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	242,000
Repayments of Unsecured Debt	$ 50,000
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)	4.60%
Secured Debt [Member] \| FNMA
Debt Instrument [Line Items]
Repayments of Secured Debt	$ 80,000
Secured Debt [Member] \| Fixed Rate Debt \| Mortgages
Debt Instrument [Line Items]
Average interest rate (percentage)	4.60%
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate	$ 476,161	882,752
Parent Company [Member]
Debt Instrument [Line Items]
Unsecured Debt	4,053,302	3,525,765
Proceeds from notes payable	869,630	$ 597,480	$ 300,000
Highlands of West Village [Member] \| Mortgages
Debt Instrument [Line Items]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate	38,300
Colonial Grand at Liberty Park [Member] \| Mortgages
Debt Instrument [Line Items]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate	350,000
Post Hyde Park [Member] \| Mortgages
Debt Instrument [Line Items]
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate	$ 179,700