Stockholders' Equity (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Stockholders' Equity [Abstract]
Aggregate common stock dividends declared	$ 18,265,000	$ 16,014,000	$ 14,510,000
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