Stock-Based Compensation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2013
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	8,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant	4,900
Status of Stock Options
Shares outstanding under option, beginning balance	473	1,267	1,714
Shares exercised	(473)	(794)	(440)
Shares forfeited			(7)
Shares outstanding under option, ending balance	0	473	1,267
Weighted average exercise price, beginning balance	$ 17.47	$ 16.59	$ 16.11
Weighted average exercise price, exercised	17.47	16.07	14.76
Weighted average exercise price, forfeited			14.52
Weighted average exercise price, ending balance	$ 0.00	$ 17.47	$ 16.59
Weighted average remaining contractual life outstanding, beginning balance		1 month	11 months	1 year 8 months
Aggregate intrinsic value, outstanding, beginning balance	$ 10.4	$ 33.7	$ 66.5
Aggregate intrinsic value, exercised	$ 10.4	14.4	19.2
Aggregate intrinsic value, outstanding, ending balance		10.4	33.7
Weighted average remaining contractual life, outstanding	0 years
Weighted average remaining contractual life option exercisable	0 years
Aggregate intrinsic value, option exercisable	$ 0.0
Proceeds from Issuance of Shares under Incentive and Share-based Compensation Plans, Including Stock Options	8.3	12.7	6.5
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	8.2	0.3	10.3
Proceeds from stock options exercised, including the tax benefit	$ 16.5	$ 13.0	$ 16.8

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Amount of tax benefit from exercise of stock options granted under share-based compensation arrangement.

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Amount of cash inflow from issuance of shares under share-based compensation arrangement. Issuance includes, but is not limited to, exercise of stock options and similar instruments.

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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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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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 by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest exercisable or convertible options. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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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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