Stock-Based Compensation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	8,000
Status of Stock Options
Number of shares authorized for grant	5,700
Shares outstanding under option, beginning balance	1,267	1,714	1,997
Shares exercised	(794)	(440)	(283)
Shares forfeited		(7)
Shares outstanding under option, ending balance	473	1,267	1,714
Weighted average exercise price, beginning balance	$ 16.59	$ 16.11	$ 15.82
Weighted average exercise price, exercised	16.07	14.76	14.04
Weighted average exercise price, forfeited		14.52
Weighted average exercise price, ending balance	$ 17.47	$ 16.59	$ 16.11
Weighted average remaining contractual life outstanding, beginning balance		11 months	1 year 8 months	2 years 7 months
Aggregate intrinsic value, outstanding, beginning balance	$ 33.7	$ 66.5	$ 80.0
Aggregate intrinsic value, exercised	14.4	19.2	13.8
Aggregate intrinsic value, outstanding, ending balance	$ 10.4	33.7	66.5
Weighted average remaining contractual life, outstanding	1 month
Share under option, option exercisable	473
Weighted average exercise price, options exercisable	$ 17.47
Weighted average remaining contractual life option exercisable	1 month
Aggregate intrinsic value, option exercisable	$ 10.4
Proceeds from stock based compensation - gross	12.7	6.5	4.0
Tax benefit (loss)	0.3	10.3	12.9
Proceeds from stock options exercised, including the tax benefit	$ 13.0	$ 16.8	$ 16.9

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Aggregate proceeds received by the entity during the annual period from exercises of stock or unit options and conversion of similar instruments granted under equity-based payment arrangements.

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Disclosure of the aggregate tax benefit realized from the exercise of stock options and the conversion of similar instruments during the annual period.

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The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested).

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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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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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Weighted average price of options that were either forfeited or expired.

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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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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of fully vested and expected to vest options that are exercisable.

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

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of non-vested options forfeited.

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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.

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

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

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