Debt (Maturities of Long-term Debt) (Details)	12 Months Ended
Dec. 31, 2018 USD ($) period	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
2019	$ 932,000
2020	1,072,000
2021	327,702,000
2022	365,000,000
2023	350,000,000
Thereafter	650,000,000
Total	1,694,706,000
Notes payable	1,685,472,000	$ 1,726,927,000
Unsecured Debt
Debt Instrument [Line Items]
Notes payable	1,495,121,000	1,535,311,000
Unsecured 500 Million 2018 Line Of Credit \| Unsecured Debt \| Line of Credit
Debt Instrument [Line Items]
Amount of line of credit	500,000,000
Notes payable	$ 205,000,000	0
Additional extension period	1 year
Number of extension periods \| period	2
Maturity extension period	6 months
$500 Million Unsecured 2015 Line of Credit \| Unsecured Debt \| Line of Credit
Debt Instrument [Line Items]
Amount of line of credit	$ 500,000,000
Notes payable	$ 0	$ 23,000,000

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of all notes and loans payable (with maturities initially due after one year or beyond the operating cycle if longer).

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