Shareholders' Equity (Summary Of Options) (Details) - Alterra Equity Award Plans [Member] - Stock Options [Member] - USD ($) $ / shares in Units, $ in Millions	8 Months Ended	12 Months Ended
Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Outstanding and exercisable, beginning balance, number of shares		22,305
Exercised, number of shares	65,934	10,787	13,636
Outstanding and exercisable, ending balance, number of shares		11,518	22,305
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Outstanding and exercisable, January 1, 2015, weighted average exercise price (dollars per share)		$ 411.98
Exercised, weighted average exercise price (dollars per share)		418.41
Outstanding and exercisable, December 31, 2015, weighted average exercise price (dollars per share)		$ 405.97	$ 411.98
Outstanding and exercisable, December 31, 2015, weighted average remaining contractual term		11 months
Outstanding and exercisable, December 31, 2015, intrinsic value		$ 5.5

Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

+ References

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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No definition available.

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- Definition

Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Number of share options (or share units) exercised during the current period.

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