Equity - Additional Information (Detail) (USD $)	1 Months Ended	9 Months Ended					1 Months Ended	9 Months Ended
	May 06, 2013	Sep. 30, 2013	Sep. 10, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011	May 29, 2013 Class A Common Stock [Member]	Sep. 30, 2013 Class A Common Stock [Member]	May 06, 2013 Class A Common Stock [Member]
Increase in Total equity		$ 650,000,000
Total equity		803,542,000		153,453,000	37,966,000	(128,939,000)
Total stock, shares authorized		200,000,000
Common stock, shares authorized		100,000,000		100,000,000
Preferred stock, shares authorized		100,000,000
Common stock, shares outstanding		28,802,000		2,927,000					2,926,651
Reverse stock split ratio	0.1							0.1
Description reverse stock split								On April 26, 2013, our board of directors approved a one-for-ten reverse stock split of our class A common stock which we effected on May 6, 2013.
Reclassification of common stock to additional paid-in capital								263,000
Additional shares issued							25,875,000
Additional shares, purchase price							$ 25.50
Issuance of additional shares, net proceeds after underwriter discounts and other offering expenses							633,800,000
Common stock and stock units, shares issued and outstanding		28,898,165
Dividend declared, amount			$ 7,800,000
Dividend per common stock		$ 0.27
Date of dividend paid		Oct. 15, 2013
Dividend payable, date of record		Sep. 30, 2013