Summary of Changes to Stockholders' Equity (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Stockholders Equity Note [Line Items]
Balance at beginning of period	$ 1,140,164
Net income	60,610	30,463
Unrealized gains on securities and derivatives, net	4,551
Other changes in equity, net	6,024
Distributions to shareholders	(23,564)	(20,232)
Balance at End of Period	$ 1,187,785