Preferred Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Series H [Member]	May 02, 2013 Series H [Member]
Class of Stock Disclosures [Abstract]
Number of preferred stock, shares authorized	50,000,000	50,000,000	50,000,000
Preferred Stock, Shares Issued					20,000
Liquidation preference per share					$ 25,000
Preferred stock dividend rate fixed percentage				5.15%
Redemption period of preferred stock				90 days
Preferred stock redemption date				Jul. 15, 2018
Impact of preferred stock redemption	$ 8	$ 8		$ 8