SCHEDULE III - REAL ESTATE AND ACCUMULATED DEPRECIATION - Note to Schedule III (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Office Properties:
Balance at beginning of year	$ 3,333,900	$ 2,548,036	$ 1,762,566
Acquisitions and improvements	409,062	894,924	911,641
Impairment on real estate	(5,400)	(11,700)	(24,258)
Real estate sold, disposed or held for sale	(405,541)	(97,360)	(101,913)
Balance at close of year	3,332,021	3,333,900	2,548,036
Accumulated Depreciation:
Balance at beginning of year	309,629	231,241	199,849
Depreciation expense	119,212	102,152	69,027
Depreciation expense - discontinued operations	0	92	(23,579)
Real estate sold, disposed or held for sale	(120,069)	(23,856)	(14,056)
Balance at close of year	$ 308,772	$ 309,629	$ 231,241