Capital Stock - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended				3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 2012 Program	Jun. 30, 2012 At The Market Transactions	Jun. 30, 2012 At The Market Transactions
Class of Stock [Line Items]
Common stock available for sale	200,000,000		200,000,000	10,000,000
Issuance of common stock, shares					295,000	1,118,900
Issuance of common stock net proceeds	$ 570,589	$ 817,736			$ 8,442	$ 31,270
Sales commissions and fees					$ 72	$ 180