Consolidated Statements of Equity (USD $) In Millions, except Share data, unless otherwise specified	Total USD ($)	Common Stock [Member]	Common Stock Including Additional Paid in Capital [Member] USD ($)	Accumulated Other Comprehensive Income (Loss) [Member] USD ($)	Retained Earnings [Member] USD ($)	Noncontrolling Interest [Member] USD ($)
Balance at Dec. 31, 2008			$ 659	$ (5)	$ 700	$ 0
Balance, shares at Dec. 31, 2008		62,575,257
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs, shares		12,477,500
Issuance of common stock, net of issuance costs, value	170		170	0	0	0
Vesting of restricted stock units, shares		128,175
Vesting of restricted stock units, value			0	0	0	0
Issuance of shares pursuant to employee stock purchase plan, shares		29,648
Issuance of shares pursuant to employee stock purchase plan, value			0	0	0	0
Noncontrolling interests' capital contributions, value			0	0	0	7
Dividends declared			0	0	76	0
Net income (loss)	89		0	0	95	(6)
Net Income (Loss) Attributable to Parent	95
Other comprehensive income (loss)			0	(1)	0	0
Balance at Dec. 31, 2009			829	(6)	719	1
Balance, shares at Dec. 31, 2009		75,210,580
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs, value	0
Vesting of restricted and performance stock units, shares		77,281
Vesting of restricted and performance stock units, value			0	0	0	0
Issuance of shares pursuant to employee stock purchase plan, shares		28,558
Issuance of shares pursuant to employee stock purchase plan, value			1	0	0	0
Noncontrolling interests' capital contributions, value			0	0	0	10
Stock-based compensation			1	0	0	0
Dividends declared			0	0	78	0
Net income (loss)	121		0	0	125	(4)
Net Income (Loss) Attributable to Parent	125
Other comprehensive income (loss)			0	1	0	0
Balance at Dec. 31, 2010	1,599		831	(5)	766	7
Balance, shares at Dec. 31, 2010		75,316,419
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock, net of issuance costs, value	0
Vesting of restricted and performance stock units, shares		17,944
Vesting of restricted and performance stock units, value			0	0	0	0
Issuance of shares pursuant to employee stock purchase plan, shares		25,435
Issuance of shares pursuant to employee stock purchase plan, value			1	0	0	0
Issuance of shares pursuant to dividend reinvestment and direct stock purchase plan, shares	3,158
Noncontrolling interests' capital contributions, value			0	0	0	(4)
Stock-based compensation			4	0	0	0
Dividends declared			0	0	80	0
Net income (loss)	147		0	0		0
Net Income (Loss) Attributable to Parent	147
Other comprehensive income (loss)	(1)
Balance at Dec. 31, 2011	$ 1,666		$ 836	$ (6)	$ 833	$ 3
Balance, shares at Dec. 31, 2011		75,362,956