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Senior Long-Term Debt And Other Debt (Narrative) (Details) (USD $)
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9 Months Ended | 9 Months Ended | 9 Months Ended | 9 Months Ended | 9 Months Ended | ||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
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Sep. 30, 2013
4.90% Unsecured Senior Notes [Member]
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Dec. 31, 2012
4.90% Unsecured Senior Notes [Member]
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Jul. 02, 2012
4.90% Unsecured Senior Notes [Member]
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Sep. 30, 2013
7.50% Unsecured Senior Debentures [Member]
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Sep. 30, 2013
6.80% Unsecured Senior Notes [Member]
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Dec. 31, 2012
6.80% Unsecured Senior Notes [Member]
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Sep. 30, 2013
3.625% Unsecured Senior Notes [Member]
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Mar. 08, 2013
3.625% Unsecured Senior Notes [Member]
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Sep. 30, 2013
5.0% Unsecured Senior Notes [Member]
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Mar. 08, 2013
5.0% Unsecured Senior Notes [Member]
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Sep. 30, 2013
3.625% And 5.0% Unsecured Senior Notes [Member]
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Sep. 30, 2013
7.20% Unsecured Senior Notes [Member]
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Sep. 30, 2013
6.25% Unsecured Senior Notes [Member]
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Sep. 30, 2013
7.35% Unsecured Senior Notes [Member]
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Dec. 31, 2012
7.35% Unsecured Senior Notes [Member]
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Sep. 30, 2013
Revolving Credit Facility [Member]
option
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Dec. 31, 2012
Revolving Credit Facility [Member]
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Jun. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]
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Sep. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]
7.20% Unsecured Senior Notes [Member]
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Jun. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]
7.20% Unsecured Senior Notes [Member]
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Apr. 16, 2007
May 1, 2013 Alterra Capital Holdings [Member]
7.20% Unsecured Senior Notes [Member]
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Sep. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]
6.25% Unsecured Senior Notes [Member]
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Jun. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]
6.25% Unsecured Senior Notes [Member]
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Sep. 27, 2010
May 1, 2013 Alterra Capital Holdings [Member]
6.25% Unsecured Senior Notes [Member]
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Sep. 30, 2013
May 1, 2013 Alterra Capital Holdings [Member]
Senior Credit Facility [Member]
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| Debt Instrument [Line Items] | |||||||||||||||||||||||||
| Debt instrument, face amount | $ 350,000,000 | $ 150,000,000 | $ 250,000,000 | $ 250,000,000 | $ 100,000,000 | $ 350,000,000 | |||||||||||||||||||
| Debt instrument, interest rate | 4.90% | 4.90% | 7.50% | 6.80% | 3.625% | 5.00% | 7.20% | 6.25% | 7.35% | 7.35% | 7.20% | 6.25% | |||||||||||||
| Net proceeds from issuance of unsecured debt | 347,200,000 | 491,200,000 | |||||||||||||||||||||||
| Debt instrument, redemption price, percentage of face amount | 100.00% | ||||||||||||||||||||||||
| Loss on extinguishment of debt | (4,100,000) | ||||||||||||||||||||||||
| Debt instrument, principal amount outstanding | 246,700,000 | 90,600,000 | |||||||||||||||||||||||
| Debt instrument, estimated fair value | 512,463,000 | 95,800,000 | 416,600,000 | ||||||||||||||||||||||
| Line of credit facility, previous capacity available | 150,000,000 | ||||||||||||||||||||||||
| Line of credit facility, capacity available | 300,000,000 | 250,000,000 | |||||||||||||||||||||||
| Number of interest rate options on revolving credit facility | 2 | ||||||||||||||||||||||||
| Line of credit facility, unused capacity, commitment fee, percent | 0.25% | ||||||||||||||||||||||||
| Borrowings outstanding under the facility | 0 | 0 | 0 | ||||||||||||||||||||||
| Credit facility, maximum capacity | 900,000,000 | ||||||||||||||||||||||||
| Credit facility, potential increased maximum capacity | 1,400,000,000 | ||||||||||||||||||||||||
| Credit facility, letters of credit issued and outstanding | $ 439,600,000 | ||||||||||||||||||||||||