Equity - Summary of Class A Common Stock Issuances (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Apr. 30, 2014	Jan. 31, 2014
Shares Issued	100,000	28,275,006	25,875,000	669,047
Net Proceeds	$ 2,800	$ 766,138	$ 633,807	$ 10,000
Class A Common Stock [Member]
Shares Issued	100,000	28,275,000			9,200,000	9,200,000	9,775,000
Net Proceeds	$ 2,758	$ 766,138			$ 252,530	$ 254,758	$ 256,092
Issue Price	$ 27.58	$ 27.14			$ 27.49	$ 27.72	$ 26.25