Fair Value of Assets and Liabilities - Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value of Financial Instruments
Senior Notes	$ 944,743	$ 944,140
Mortgage notes payable, net	$ 182,083	183,100
Senior unsecured notes, 3.75% interest rate, due in 2019
Fair Value of Financial Instruments
Interest rate (as a percent)	3.75%
Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Interest rate (as a percent)	5.875%
Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Interest rate (as a percent)	4.00%
Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Interest rate (as a percent)	4.05%
Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Interest rate (as a percent)	5.72%
Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Interest rate (as a percent)	4.22%
Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Interest rate (as a percent)	4.80%
Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Interest rate (as a percent)	5.877%
Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Interest rate (as a percent)	7.00%
Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Interest rate (as a percent)	8.15%
Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Interest rate (as a percent)	4.26%
Carrying Amount
Fair Value of Financial Instruments
Mortgage notes payable, net	$ 1,126,826	1,127,240
Carrying Amount | Senior unsecured notes, 3.75% interest rate, due in 2019
Fair Value of Financial Instruments
Senior Notes	348,381	348,096
Carrying Amount | Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Senior Notes	300,318	300,232
Carrying Amount | Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	296,044	295,812
Carrying Amount | Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Senior Notes	64,343	64,293
Carrying Amount | Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Senior Notes	35,786	36,085
Carrying Amount | Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	27,742	27,906
Carrying Amount | Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Senior Notes	25,394	25,501
Carrying Amount | Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	13,560	13,620
Carrying Amount | Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Mortgage notes payable, net	8,291	8,391
Carrying Amount | Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	3,823	4,111
Carrying Amount | Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable, net	3,144	3,193
Fair Value
Fair Value of Financial Instruments
Mortgage notes payable, net	1,150,400	1,164,606
Fair Value | Senior unsecured notes, 3.75% interest rate, due in 2019
Fair Value of Financial Instruments
Senior Notes	351,673	354,993
Fair Value | Senior unsecured notes, 5.875% interest rate, due in 2046
Fair Value of Financial Instruments
Senior Notes	314,464	320,416
Fair Value | Senior unsecured notes, 4.000% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	301,044	302,655
Fair Value | Mortgage note payable, 4.050% interest rate, due in 2030
Fair Value of Financial Instruments
Senior Notes	63,918	65,198
Fair Value | Mortgage note payable, 5.720% interest rate, due in 2020
Fair Value of Financial Instruments
Senior Notes	35,812	36,332
Fair Value | Mortgage note payable, 4.220% interest rate, due in 2022
Fair Value of Financial Instruments
Senior Notes	28,081	28,432
Fair Value | Mortgage note payable, 4.800% interest rate, due in 2023
Fair Value of Financial Instruments
Senior Notes	25,488	25,904
Fair Value | Mortgage note payable, 5.877% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	14,345	14,565
Fair Value | Mortgage note payable, 7.000% interest rate, due in 2019
Fair Value of Financial Instruments
Mortgage notes payable, net	8,410	8,555
Fair Value | Mortgage note payable, 8.150% interest rate, due in 2021
Fair Value of Financial Instruments
Mortgage notes payable, net	4,014	4,340
Fair Value | Mortgage note payable, 4.260% interest rate, due in 2020
Fair Value of Financial Instruments
Mortgage notes payable, net	$ 3,151	$ 3,216