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Common Equity and Earnings Per Share - Additional Information (Detail) (USD $)
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9 Months Ended | 12 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | |
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Sep. 30, 2013
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Dec. 31, 2012
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Sep. 30, 2013
Class A
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Sep. 30, 2013
Class A
2013 Plan
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Sep. 30, 2013
Class A
2013 Plan
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Sep. 30, 2013
Class B
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| Class of Stock | ||||||
| Common stock, votes per share | One | Ten | ||||
| Payments of dividends, common stock | ||||||
| Preferred stock, shares issued | ||||||
| Preferred stock, shares outstanding | ||||||
| Shares issuable under stock options excluded from calculation of diluted earnings per share | 600,000 | 600,000 | ||||