CONSOLIDATED STATEMENTS OF CASH FLOWS $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
CASH FLOWS FROM OPERATING ACTIVITIES
Net income	$ 81,921	$ 56,859	$ 40,052
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	108,652	95,702	80,722
Amortization of above/below market leases	(541)	(520)	(556)
Amortization of deferred financing costs	1,333	1,246	1,897
Gain on real estate disposal		(36)	(1,208)
Share-based compensation	8,892	7,114	6,125
Bad debt expense	398	201	648
Changes in operating assets and liabilities:
Accounts receivable	(13,199)	(2,146)	(599)
Deferred rent receivable	(7,083)	(7,799)	(4,245)
Deferred leasing costs	(14,395)	(17,555)	(21,390)
Other assets	(8,819)	(5,535)	(4,459)
Accounts payable and accrued expenses	4,910	6,532	(4,160)
Unearned revenue, prepaid rent and other liabilities	8,696	9,561	7,350
Deferred rent payable	(240)	(1,051)	(661)
Net cash provided by operating activities	170,525	142,573	99,516
CASH FLOWS FROM INVESTING ACTIVITIES
Tenant improvements	(5,878)	(7,736)	(5,893)
Real estate improvements	(293,359)	(122,196)	(100,299)
Acquisition of Reston Campus Expansion	(64,965)
Proceeds from real estate disposal		2,399
Net cash used in investing activities	(364,202)	(127,533)	(106,192)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from exercise of stock options	1,290	1,393	2,354
Offering costs			(147)
Proceeds from revolving credit facility	299,500	87,000	88,250
Payments on revolving credit facility	(247,750)	(163,250)	(44,000)
Proceeds from unsecured term loan	250,000	150,000	100,000
Repayments of mortgage loans payable			(58,250)
Payments of loan fees and costs	(2,981)	(2,315)	(1,000)
Payments to net settle equity awards		(4,256)	(1,410)
Dividends and distributions	(108,807)	(87,420)	(73,772)
Net cash provided by (used in) financing activities	191,252	(18,848)	12,025
Net change in cash and cash equivalents	(2,425)	(3,808)	5,349
Cash and cash equivalents, beginning of year	6,854	10,662	5,313
Cash and cash equivalents, end of year	4,429	6,854	10,662
SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION
Cash paid for interest, net of capitalized amounts	11,528	6,219	4,072
NON-CASH INVESTING AND FINANCING ACTIVITY
Construction costs payable capitalized to real estate	28,805	37,033	16,142
Accrual of dividends and distributions	$ 41,849	$ 28,104	$ 22,355