Notes and Bonds Payable (Details)							3 Months Ended	9 Months Ended
	Nov. 01, 2017 USD ($)	Oct. 02, 2017 USD ($) ft²	Sep. 28, 2017 USD ($) ft²	Jun. 13, 2017 USD ($) ft²	May 01, 2017 USD ($) ft²	Mar. 06, 2017 USD ($) ft²	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($) ft²	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Notes and bonds payable								$ 1,166,060,000		$ 1,264,370,000
Aggregate redemption price of debt								4,654,000	$ 36,786,000
Accrued interest paid								42,136,000	$ 43,879,000
Line of Credit | Unsecured Credit Facility Due 2020
Debt Instrument [Line Items]
Notes and bonds payable								$ 0		107,000,000
Effective interest rate (percent)								2.23%
Medium-term Notes | Term Loan Due 2019
Debt Instrument [Line Items]
Notes and bonds payable								$ 149,667,000		149,491,000
Effective interest rate (percent)								2.43%
Senior Notes | Senior Notes due 2021, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable								$ 397,653,000		397,147,000
Effective interest rate (percent)								5.97%
Senior Notes | Senior Notes due 2023, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable								$ 247,601,000		247,296,000
Effective interest rate (percent)								3.95%
Senior Notes | Senior Notes due 2025, net of discount and issuance costs
Debt Instrument [Line Items]
Notes and bonds payable								$ 247,987,000		247,819,000
Effective interest rate (percent)								4.08%
Mortgages
Debt Instrument [Line Items]
Notes and bonds payable								$ 123,152,000		$ 115,617,000
Effective interest rate (percent)								5.06%
MINNESOTA | Mortgages | 6.5% Mortgage Note Payable
Debt Instrument [Line Items]
Mortgage note, interest rate (percent)					6.50%
Amount of debt repaid					$ 200,000
Square footage encumbered by mortgage payable (sqft) | ft²					60,476
TENNESSEE | Mortgages | 5.55% Mortgage Note Payable
Debt Instrument [Line Items]
Mortgage note, interest rate (percent)			5.55%
Amount of debt repaid			$ 1,300,000
Square footage encumbered by mortgage payable (sqft) | ft²			75,000
Real Estate Acquisition
Debt Instrument [Line Items]
Square footage | ft²								215,416
Mortgage notes payable assumed								$ 12,100,000
Fair value adjustments								400,000
Real Estate Acquisition | MINNESOTA
Debt Instrument [Line Items]
Square footage | ft²						34,608
Mortgage notes payable assumed						$ 0
Real Estate Acquisition | DISTRICT OF COLUMBIA
Debt Instrument [Line Items]
Effective interest rate (percent)				4.27%
Mortgage note, interest rate (percent)				4.69%
Square footage | ft²				62,379
Mortgage notes payable assumed				$ 12,100,000
Fair value adjustments								$ 400,000
Subsequent Event | Senior Notes | Senior Notes due 2021, net of discount and issuance costs
Debt Instrument [Line Items]
Amount of debt repaid	$ 100,000,000.0
Aggregate redemption price of debt	113,400,000
Principal amount redeemed	100,000,000
Accrued interest paid	1,700,000
Make-whole payment	11,700,000
Write off of unaccreted discount and unamortized costs	$ 500,000
Subsequent Event | OHIO | Mortgages | 5.53% Mortgage Note Payable
Debt Instrument [Line Items]
Mortgage note, interest rate (percent)		553.00%
Amount of debt repaid		$ 200,000
Square footage encumbered by mortgage payable (sqft) | ft²		73,331
Forecast | Senior Notes | Senior Notes due 2021, net of discount and issuance costs
Debt Instrument [Line Items]
Loss on extinguishment of debt							$ 12,200,000