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Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
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Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenue |  | $ 705,157 | $ 686,284 | $ 664,484 | $ 661,151 | $ 655,229 | $ 657,712 | $ 656,907 | $ 632,228 | $ 2,717,076 | $ 2,602,076 | $ 2,550,820
Expenses |  |  |  |  |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  |  |  |  | 645,649 | 617,547 | 694,403
Total expenses |  |  |  |  |  |  |  |  |  | 1,791,579 | 1,693,966 | 1,723,253
Other income (expense) |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  |  | (378,168) | (374,481) | (412,849)
Gains on sales of real estate |  |  |  |  |  |  |  |  |  | 182,356 | 7,663 | 80,606
Net income |  |  |  |  |  |  |  |  |  | 712,563 | 562,481 | 569,977
Income from unconsolidated joint ventures |  |  |  |  |  |  |  |  |  | 2,222 | 11,232 | 8,074
Gain on sale of investment in unconsolidated joint venture |  |  |  |  |  |  |  |  |  | 0 | 0 | 59,370
Straight-line rent adjustments |  |  |  |  |  |  |  |  |  | 51,900 | 54,800 | 31,700
Mortgage and notes payable, net |  | 2,964,572 |  |  |  | 2,979,281 |  |  |  | 2,964,572 | 2,979,281 |
Above and below market rent adjustments, net |  |  |  |  |  |  |  |  |  | 23,800 | 23,500 | 30,200
Gain on sale of investment in unconsolidated joint venture |  |  |  |  |  |  |  |  |  | 0 | 0 | 55,707
Unconsolidated Joint Ventures [Member] |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total revenue | [1] |  |  |  |  |  |  |  |  | 271,951 | 222,517 | 177,182
Expenses |  |  |  |  |  |  |  |  |  |  |  |
Operating |  |  |  |  |  |  |  |  |  | 106,610 | 90,542 | 76,741
Depreciation and amortization | [2] |  |  |  |  |  |  |  |  | 103,079 | 57,079 | 44,989
Total expenses |  |  |  |  |  |  |  |  |  | 209,689 | 147,621 | 121,730
Other income (expense) |  |  |  |  |  |  |  |  |  |  |  |
Interest expense |  |  |  |  |  |  |  |  |  | (71,308) | (46,371) | (34,016)
Gains on sales of real estate |  |  |  |  |  |  |  |  |  | 16,951 | 0 | 0
Net income |  |  |  |  |  |  |  |  |  | 7,905 | 28,525 | 21,436
Company's share of net income |  |  |  |  |  |  |  |  |  | 8,084 | 18,439 | 9,873
Basis differential | [3] |  |  |  |  |  |  |  |  | (5,862) | (7,207) | (1,799)
Income from unconsolidated joint ventures |  |  |  |  |  |  |  |  |  | 2,222 | 11,232 | 8,074
Gain on sale of investment in unconsolidated joint venture |  |  |  |  |  |  |  |  |  | 0 | 0 | 59,370
Straight-line rent adjustments |  |  |  |  |  |  |  |  |  | 15,900 | 21,700 | 18,100
Mortgage and notes payable, net |  | $ 2,017,609 |  |  |  | $ 1,437,440 |  |  |  | 2,017,609 | 1,437,440 |
The Metropolitan Square Associates LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |
Expenses |  |  |  |  |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  |  |  |  | 22,400 |  |
The Metropolitan Square Associates LLC [Member] | Boston Properties, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |
Expenses |  |  |  |  |  |  |  |  |  |  |  |
Depreciation and amortization |  |  |  |  |  |  |  |  |  | 4,500 |  |
Colorado Center [Member] |  |  |  |  |  |  |  |  |  |  |  |
Other income (expense) |  |  |  |  |  |  |  |  |  |  |  |
Straight-line rent adjustments |  |  |  |  |  |  |  |  |  | 2,400 | 1,900 | 1,400
Above and below market rent adjustments, net |  |  |  |  |  |  |  |  |  | $ 1,600 | $ 2,900 | $ 900
[1] | Includes straight-line rent adjustments of approximately $15.9 million, $21.7 million and $18.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.
[2] | During the year ended December 31, 2018, the joint venture that owns Metropolitan Square in Washington, DC, commenced a renovation project and recorded accelerated depreciation expense of approximately $22.4 million related to the remaining book value of the assets to be replaced. The Company's share of the accelerated depreciation expense totaled approximately $4.5 million.
[3] | Includes straight-line rent adjustments of approximately $2.4 million, $1.9 million and $1.4 million for the years ended December 31, 2018, 2017 and 2016, respectively. Also includes net above-/below-market rent adjustments of approximately $1.6 million, $2.9 million and $0.9 million for the years ended December 31, 2018, 2017 and 2016, respectively.

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