Consolidated Statements of Stockholders' Equity (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common stock (in dollars per share)	$ 1.65	$ 1.60	$ 1.56
Stock issued during period, shares	5,493,595	29,013	40,460
ATM equity program, shares	3,758,362	6,956,992	4,282,298
Conversion of 2028 Notes, shares		2,407,911
Discounted stock purchase program, shares	100,161	322,084	1,689,160
Restricted common stock, shares	360,080	290,181	373,913
Series C Preferred Stock
Series C preferred stock (in dollars per share)			$ 0.53776
Redemption of shares of Series C Preferred Stock			3,680,000
Series D Depositary Share
Series D preferred stock (in dollars per share)	$ 1.65625	$ 1.65625	$ 1.34340
Stock issued during period, shares			11,500,000
Series E Depositary Share
Series D preferred stock (in dollars per share)	$ 1.42500	$ 0.77188
Stock issued during period, shares		11,500,000