Stockholders' Equity (Details) (USD $)	12 Months Ended			3 Months Ended	12 Months Ended			0 Months Ended	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Ordinary Dividend [Member]	Dec. 31, 2012 Ordinary Dividend [Member]	Dec. 31, 2011 Ordinary Dividend [Member]	Dec. 31, 2010 Ordinary Dividend [Member]	Dec. 12, 2012 Special Dividend [Member]	Dec. 31, 2012 Special Dividend [Member]
Class of Stock [Line Items]
Aggregated regular quarterly dividends on common stock	$ 285,521,000	$ 18,265,000	$ 16,014,000		$ 34,877,000	$ 18,265,000	$ 16,014,000	$ 250,644,000
Common stock dividends paid				$ 0.1875					$ 3.75
Description of common stock voting rights	Each share of common stock entitles the holder to one vote on all matters on which holders are permitted to vote, including the election of directors. There are no cumulative voting rights.
Description of common stock subscription rights	The Company’s common stock has no preemptive or conversion rights or other subscription rights.
Common stock, redemption provision	0
Common stock, sinking fund	$ 0