Compensation Plans Stock Option Grants Under the Equity Incentive Compensation Plan (Details) (Employee Stock Option [Member], USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Stock Option [Member]
Compensation plans
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options	$ 0
Shares
Outstanding, start of year	39,088	267,846	508,214
Exercised	(39,088)	(228,758)	(240,368)
Forfeited	0	0	0
Outstanding, end of year	0	39,088	267,846
Vested and exercisable at end of year	0	39,088	267,846
Weighted Average Exercise Price
Outstanding, start of year (in dollars per share)	$ 20.87	$ 14.95	$ 13.86
Exercised (in dollars per share)	$ 20.87	$ 13.92	$ 12.65
Forfeited (in dollars per share)	$ 0	$ 0	$ 0
Outstanding, end of year (in dollars per share)	$ 0	$ 20.87	$ 14.95
Vested and exercisable at end of year (in dollars per share)	$ 0	$ 20.87	$ 14.95
Aggregate Intrinsic Value
Exercised	1,993,726	12,326,994	11,842,575
Outstanding	0	2,432,837	9,983,177
Vested and exercisable at end of year	$ 0	$ 2,432,837	$ 9,983,177

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Unrecognized cost of unvested options awarded to employees as compensation.

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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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

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

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

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The number of exercisable share options (fully vested and expected to vest) that may be converted as of the balance sheet date.

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As of the balance sheet date, the weighted-average exercise price (at which grantees can acquire the shares reserved for issuance) for exercisable stock options that are fully vested or expected to vest.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of fully vested and expected to vest options outstanding.

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

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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

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