Consolidated Statement of Changes in Stockholders' Equity and Noncontrolling Interests (Unaudited) - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income	Noncontrolling Interests	Redeemable Noncontrolling Interests	7.00% Series D Cumulative Convertible Preferred Stock Preferred Stock
Beginning balance (shares) at Dec. 31, 2017		99,783,686
Beginning balance at Dec. 31, 2017	$ 6,471,660	$ 998	$ 5,824,258	$ 0	$ 50,024	$ 521,994		$ 74,386
Increase (Decrease) in Stockholders' Equity
Net Income	141,304			135,630		5,674
Total other comprehensive income	975				975
Reclassification of cumulative net unrealized gains on non-real estate investments	90,750			140,521	(49,771)
Distributions to noncontrolling interests	(5,709)					(5,709)
Sale of and contributions from noncontrolling interests	6,579		0			6,579
Issuances of common stock (shares)		843,600
Issuance of common stock	99,369	$ 8	99,361
Issuances pursuant to stock plan (in shares)		69,075
Issuances pursuant to stock plan	11,489	$ 1	11,488
Dividends declared on common stock	(91,980)			(91,980)
Dividends declared on preferred stock	(1,302)			(1,302)
Distributions in excess of earnings	0		182,869	(182,869)
Ending balance (shares) at Mar. 31, 2018		100,696,361
Ending balance at Mar. 31, 2018	6,723,135	$ 1,007	$ 6,117,976	$ 0	$ 1,228	$ 528,538		$ 74,386
Beginning balance at Dec. 31, 2017	11,509						$ 11,509
Increase (Decrease) in Temporary Equity [Roll Forward]
Net Income							214
Redemption of noncontrolling interest							(1,297)
Distributions to noncontrolling interests							(214)
Ending balance at Mar. 31, 2018	$ 10,212						$ 10,212