Consolidated Balance Sheets (Parenthetical) - $ / shares	Jun. 30, 2018	Dec. 31, 2017
Common stock, par value	$ 0.10	$ 0.10
Common stock, shares authorized	111,363,800	113,663,800
Common stock, shares issued	36,896,969	35,488,068
Common stock, shares outstanding	36,896,969	35,488,068
Excess stock, par value	$ 0.10	$ 0.10
Excess stock, shares authorized	3,000,000	3,000,000
Excess stock, shares issued
Excess stock, shares outstanding
Series B Cumulative Redeemable Preferred Stock [Member]
Percentage rate on cumulative redeemable preferred stock	8.00%	8.00%
Cumulative redeemable preferred stock, par value	$ 0.10	$ 0.10
Cumulative redeemable preferred stock, shares authorized	4,000,000	4,000,000
Cumulative redeemable preferred stock, shares issued	3,801,200	3,801,200
Cumulative redeemable preferred stock, shares outstanding	3,801,200	3,801,200
Series C Cumulative Redeemable Preferred Stock [Member]
Percentage rate on cumulative redeemable preferred stock	6.75%	6.75%
Cumulative redeemable preferred stock, par value	$ 0.10	$ 0.10
Cumulative redeemable preferred stock, shares authorized	5,750,000	5,750,000
Cumulative redeemable preferred stock, shares issued	5,750,000	5,750,000
Cumulative redeemable preferred stock, shares outstanding	5,750,000	5,750,000
Series D Cumulative Redeemable Preferred Stock [Member]
Percentage rate on cumulative redeemable preferred stock	6.375%	0.00%
Cumulative redeemable preferred stock, par value	$ 0.10	$ 0.00
Cumulative redeemable preferred stock, shares authorized	2,300,000	0
Cumulative redeemable preferred stock, shares issued	2,000,000	0
Cumulative redeemable preferred stock, shares outstanding	2,000,000	0