Equity offerings (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended	9 Months Ended
	May 02, 2013	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Equity offering [Abstract]
Common stock, shares issued	11,500,000	68,202,000		56,331,000
Common stock issuance price per share	$ 20.00
Net proceeds from issuance of common shares	$ 220,300	$ 220,300
Underwriting discount per share on public offering	$ 0.80
Amount outstanding	$ 192,500	$ 90,000