Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
	Aug. 03, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2008
Class of Stock [Line Items]
Preferred stock, par value		$ 0.01	$ 0.01
Preferred stock, shares authorized		9,940,000	9,940,000
Net proceeds from issuance of preferred stock offerings		$ 581
Common Stock [Member]
Class of Stock [Line Items]
Shares issued		0	0	0
Series A Preferred Stock [Member]
Class of Stock [Line Items]
Fixed dividend rate on preferred stock		7.00%
Preferred stock liquidation preference (per share)		$ 1,000	$ 1,000
End date of fixed dividend rate on Preferred stock		2022-02
Series A Preferred Stock [Member] | LIBOR [Member]
Class of Stock [Line Items]
Floating rate basis spread		4.82%
Series A Preferred Stock [Member] | Minimum [Member]
Class of Stock [Line Items]
Preferred stock earliest redemption date		Feb. 01, 2022
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Fixed dividend rate on preferred stock		6.00%
Preferred stock liquidation preference (per share)		$ 1,000	$ 1,000
Series B Preferred Stock [Member] | Minimum [Member]
Class of Stock [Line Items]
Preferred stock earliest redemption date		Sep. 01, 2017
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Depositary shares issued	24,000,000
Depositary shares preferred stock ownership interest (per share)	2.50%
Fixed dividend rate on preferred stock	6.00%	6.00%
Preferred stock, par value	$ 0.01
Preferred stock liquidation preference (per share)	1,000	$ 1,000
Depositary share, liquidation preference equivalent (per share)	$ 25
Series C Preferred Stock [Member] | Minimum [Member]
Class of Stock [Line Items]
Preferred stock earliest redemption date					Dec. 01, 2020