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Senior Unsecured Notes (Narrative) (Details) (USD $)
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6 Months Ended | |
|---|---|---|
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Jun. 30, 2013
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Jun. 30, 2012
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| Debt Instrument [Line Items] | ||
| Proceeds from the sale of senior unsecured notes | $ 268,928,000 | $ 299,403,000 |
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3.150% Senior Unsecured Note, Due May 15, 2023 [Member]
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| Debt Instrument [Line Items] | ||
| Face amount of senior unsecured notes | 275,000,000 | |
| Interest rate of Senior Unsecured Notes | 3.15% | |
| Maturity date of the Senior Unsecured Notes | May 15, 2023 | |
| Proceeds from the sale of senior unsecured notes | $ 266,500,000 | |