Capital Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended			12 Months Ended
	Apr. 02, 2011	Mar. 18, 2011	Jan. 05, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Class of Stock [Line Items]
Common stock available for sale				100,000,000	100,000,000
Common stock par value				$ 0.001	$ 0.001
Preferred stock authorized to issue				10,000,000	10,000,000
Preferred stock par value				$ 0.001	$ 0.001
Issuance of common stock	2,175,000	14,500,000	11,500,000
Issuance of common stock, offering price	$ 28.50	$ 28.50	$ 28.75
Issuance of common stock net proceeds	$ 61,985	$ 407,606	$ 325,707	$ 860,602	$ 271,373	$ (223)
Amount charged to additional paid in capital in connection with the issuance						(223)
Underwriters' Overallotment Option [Member]
Class of Stock [Line Items]
Issuance of common stock			1,500,000
CEO Program [Member]
Class of Stock [Line Items]
Common stock available for sale				5,000,000
Issuance of common stock				2,361,500	2,314,600
Sales commissions and fees				66,221	65,731
Amount charged to additional paid in capital in connection with the issuance				$ 1,037	$ 1,530