Fair Value of Financial Instruments - Fair Value, Recurring and Nonrecurring (Details) - USD ($)	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Derivative assets	$ 26,988,000	$ 12,586,000
Derivative liabilities	0	265,000
Gross unrealized gain	42,000
Gross unrealized losses	28,000
Unsecured Debt \| Registered Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Face amount	400,000,000.0
Amortization of discount	667,000	722,000
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
U.S. Government securities	148,315,000	0
Due in 1 year	6,107,000
Due in 1 year through 5 years	142,208,000
Total	148,315,000
Carrying Value \| Unsecured Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	1,984,333,000	1,974,278,000
Carrying Value \| Secured Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	365,519,000	464,311,000
Carrying Value \| In-Substance Defeased Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	139,003,000	0
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions
U.S. Government securities	148,328,000	0
Due in 1 year	6,120,000
Due in 1 year through 5 years	142,208,000
Total	148,328,000
Fair Value \| Unsecured Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	1,928,411,000	1,960,560,000
Fair Value \| Secured Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	353,810,000	458,441,000
Fair Value \| In-Substance Defeased Debt
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Debt	136,515,000	0
Non-Real Estate Investment
Fair Value, Balance Sheet Grouping, Financial Statement Captions
U.S. Government securities	2,713,000	0
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Derivative assets	0	0
Derivative liabilities	0	0
Level 1 \| Non-Real Estate Investment
Fair Value, Balance Sheet Grouping, Financial Statement Captions
U.S. Government securities	0	0
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Derivative assets	26,988,000	12,586,000
Derivative liabilities	0	265,000
Level 2 \| Non-Real Estate Investment
Fair Value, Balance Sheet Grouping, Financial Statement Captions
U.S. Government securities	2,713,000	0
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions
Derivative assets	0	0
Derivative liabilities	0	0
Level 3 \| Non-Real Estate Investment
Fair Value, Balance Sheet Grouping, Financial Statement Captions
U.S. Government securities	$ 0	$ 0

Amount of unrealized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date.

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

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Amount, after accumulated amortization, of debt discount.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value portion of notes payable.

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