Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2016	Dec. 31, 2015
Statement Of Financial Position [Abstract]
Preferred stock, liquidation preference per share	$ 50	$ 50
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	100,000,000	100,000,000
Preferred stock, shares issued	1,565,000	1,565,000
Preferred stock, shares outstanding	1,565,000	1,565,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares issued	528,671,000	524,512,000
Common stock, shares outstanding	528,671,000	524,512,000