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Stockholders' Equity (Details) (USD $)
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12 Months Ended | ||
|---|---|---|---|
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Dec. 31, 2011
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Dec. 31, 2010
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Dec. 31, 2009
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| 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 | ||