Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Series D Preferred Stock	Series E Preferred Stock	Total Stockholders’ Equity	Total Stockholders’ Equity Series D Preferred Stock	Total Stockholders’ Equity Series E Preferred Stock	Preferred Stock Series D Preferred Stock	Preferred Stock Series E Preferred Stock	Common Stock	Capital in Excess of Par Value	Capital in Excess of Par Value Series E Preferred Stock	Retained Earnings (Loss)	Retained Earnings (Loss) Series D Preferred Stock	Retained Earnings (Loss) Series E Preferred Stock	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interests
Balances at at Dec. 31, 2012	$ 2,297,585			$ 2,296,285			$ 287,500	$ 0	$ 1,117	$ 2,101,002		$ (90,952)			$ (2,382)	$ 1,300
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	160,085			160,145								160,145				(60)
Dividends declared and paid:
Preferred stock dividends declared and paid		$ (19,047)	$ (8,876)		$ (19,047)	$ (8,876)							$ (19,047)	$ (8,876)
Common stock dividends declared and paid	(174,162)			(174,162)					4	14,941		(189,107)
Issuance of stock	744		277,644	744		277,644		287,500		744	$ (9,856)
Issuance of common stock - stock purchase program	10,461			10,461					3	10,458
Issuance of common stock - ATM equity program	242,418			242,418					70	242,348
Issuance of common stock - conversion of 2028 Notes	85,224			85,224					24	85,200
Issuance of shares of restricted common stock	(210)			(210)					3	(213)
Equity component of convertible debt	(93,450)			(93,450)						(93,450)
Stock issuance costs	(3,774)			(3,774)						(3,774)
Amortization of deferred compensation	6,715			6,715						6,715
Amortization of interest rate hedges	438			438											438
Fair value forward starting swaps	(3,141)			(3,141)											(3,141)
Unrealized gain – commercial mortgage residual interests	(438)			(438)											(438)
Valuation adjustments – available-for-sale securities	69			69											69
Noncontrolling interests										(949)					949
Balances at at Dec. 31, 2013	2,778,285			2,777,045			287,500	287,500	1,221	2,353,166		(147,837)			(4,505)	1,240
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	191,170			190,601								190,601				569
Dividends declared and paid:
Preferred stock dividends declared and paid		(19,047)	(16,387)		(19,047)	(16,387)							(19,047)	(16,387)
Common stock dividends declared and paid	(192,711)			(192,711)					3	11,443		(204,157)
Issuance of stock	209,240			209,240					55	209,185
Issuance of common stock - stock purchase program	3,371			3,371					1	3,370
Issuance of common stock - ATM equity program	137,115			137,115					38	137,077
Issuance of shares of restricted common stock	(309)			(309)					4	(313)
Stock issuance costs	(10,683)			(10,683)						(10,683)
Amortization of deferred compensation	8,433			8,433						8,433
Amortization of interest rate hedges	1,129			1,129											1,129
Fair value forward starting swaps	(6,312)			(6,312)											(6,312)
Unrealized gain – commercial mortgage residual interests	875			875											875
Realized gain – commercial mortgage residual interests	163			163											163
Valuation adjustments – available-for-sale securities	111			111											111
Realized gain – available-for-sale securities	(119)			(119)											(119)
Distributions to noncontrolling interest	(1,232)															(1,232)
Balances at at Dec. 31, 2014	3,083,092			3,082,515			287,500	287,500	1,322	2,711,678		(196,827)			(8,658)	577
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net earnings	197,961			197,836								197,836				125
Dividends declared and paid:
Preferred stock dividends declared and paid		$ (19,047)	$ (16,387)		$ (19,047)	$ (16,387)							$ (19,047)	$ (16,387)
Common stock dividends declared and paid	(221,811)			(221,811)					2	6,886		(228,699)
Issuance of stock	991			991						991
Issuance of common stock - stock purchase program	455			455						455
Issuance of common stock - ATM equity program	325,084			325,084					86	324,998
Issuance of shares of restricted common stock	(309)			(309)					2	(311)
Stock issuance costs	(4,178)			(4,178)						(4,178)
Amortization of deferred compensation	8,679			8,679						8,679
Amortization of interest rate hedges	1,902			1,902											1,902
Fair value forward starting swaps	(13,369)			(13,369)											(13,369)
Unrealized gain – commercial mortgage residual interests	(585)			(585)											(585)
Realized gain – commercial mortgage residual interests	246			246											246
Valuation adjustments – available-for-sale securities	112			112											112
Contributions from noncontrolling interest	334															334
Distributions to noncontrolling interest	(362)															(362)
Sale of noncontrolling interest	(415)															(415)
Balances at at Dec. 31, 2015	$ 3,342,393			$ 3,342,134			$ 287,500	$ 287,500	$ 1,412	$ 3,049,198		$ (263,124)			$ (20,352)	$ 259